2.1 Green advertisements

Boomed in the late 1980's, green advertising served as a powerful tool in bringing information relevant to environmental sustainability into the market. It also filled the gap in information by overseeing environmental issues within marketing activities and strategies (^{Kardos et al., 2019}). Green advertising is any ad contributing to environmental protection by providing a description of specific environment-related activities or showing its environmental impact (^{Haytko & Matulich, 2008}). It is becoming an important technique used by many destinations and organisations to spread knowledge and increase public awareness about eco-friendly practices (^{Fernando et al., 2016}).

As argued by ^{Abd-Rahim, Zukni, Ahmad & Lyndon (2012}), green ads encourage sustainable behaviour and, at times, reflect the social responsibility of any company, destination, hotel or any other institutions. It aims at minimising negative environmental and social impacts resulting from damaging practices done by tourists in many tourist destinations (^{Haytko & Matulich, 2008}). Thus, green ad campaigns not only disseminate the concept of a healthier environment but also embed the development of good quality of life while providing a competitive advantage for the destination (^{Kao & Du, 2020}).

^{Rashid (2009}) noted that a green ad is an advertisement that promotes a brand, product, service, behaviour, attitude or thoughts that ultimately helps in reducing environmental harm and minimising the extent of our ecological imprint. Green ads increase the publics' knowledge and consciousness towards the environment and directing their attention to its problems regarding human interactions and negative practices of tourism activities (^{Chan, 2004}).

2.2 Celebrity endorsement in advertisements

Celebrity endorsement consists of two terms. Celebrity refers to people who are famous and well known to many individuals in the society and have a wide reputation in a local or global scale for his/ her achievements in their work area (^{Yang, 2018}). Celebrities can also be described as reference groups who have a significant influence on an individual's evaluations, intentions and behaviours (^{Min, Chang, Jai & Ziegler 2019}).

Nowadays, in the era of social networking and electronic media advertisements, there are numerous types of celebrities, depending on the work area (^{Kim et al., 2019}). This includes movie stars, sports celebrities, musicians, politicians and others (^{Yen & Teng, 2015}). According to ^{Kadekova & Holienčinová (2018}), celebrities are the original influencers who provide an imperative source of meaning because they are seen as authentic, what differ from other groups of influencers whose star power derives from self-promotion via digital content. As noted by ^{Schouten, Janssen & Verspaget (2020}), anyone can be a social influencer through the creation of content via social media channels, but not anyone can be a celebrity.

'Endorsement' is defined as the channel of communication wherein the spokesperson of a particular brand is represented by celebrities who extend their popularity as well as personality and therefore certify the claims and position of the aforementioned brand (^{Yang, 2018}). With the above, the implementation of celebrity endorsement prevails in promoting brands, products or even services to attract different audiences (^{Chan, Lee & Wong, 2018}).

Many researchers have developed definitions for celebrity endorsement. ^{Lee et al. (2008}) described it as an unnoticeable state of motivation, encouragement, or interest in an associated activity. It has also been designated by ^{Chan et al. (2018}) as an induced way used to stimulate tourists to visit a particular destination. Nearly most studies have agreed to the concept of using celebrities in advertisements to target potential customers for specific purposes (^{Carroll, 2009}; ^{Yang, 2018}; ^{Kim et al., 2019}).

Currently, due to digitalisation, the main challenge for many destinations is to create advertising that is easily observed and remembered in today's highly busy environment. Celebrity support has become a vogue in the field of advertisements in recent years (^{Armielia, 2018}). Several previous studies have investigated different perspectives on celebrity endorsements concerning their effectiveness in advertising (^{Kumari, 2018}). Earlier studies shown in Table (1) evidenced that celebrities play an important role in influencing consumers' preferences, behaviours, attitudes, intentions and many other purposes.

Table 1 Results from previous literature 

Based on the above, it was proved that using celebrities in ads are now not only aimed at publicising products or brands etc., but they are currently employed for numerous purposes by several bodies, organisations and destinations in a much broader sense (^{Panda & Thakkar, 2017}).

2.3 Green ads and celebrity endorsement

Over the years, public and organisational support to protect the environment has increased drastically. Many tourist destinations and companies have perceived well the advantages of being environmentally responsible. Several destinations and organisations have developed green advertising as a marketing tool to improve the environmental performance of consumers (^{Kardos et al., 2019}). World leaders have supported eco-friendly practices in terms of tourism activities to make tourist destination more sustainable, especially considering that the competitiveness of any destination enormously relies on the level of sustainability (^{Khatter, McGrath, Pyke, White & Lockstone-Binney, 2019}).

The idea behind using celebrities in green ad campaigns is to capture people's attention to the sustainable behaviours that must be practised in resorts and destinations while also encouraging them to go green not only during tourism activities but also in their daily lives (^{Blasche & Ketelaar, 2015}). The eventual goal is to guide the behavioural intentions of individuals towards the environment in a positive manner which will, in turn, assist in achieving sustainability for many destinations (^{Domazet & Kovačević, 2018}). ^{Alniacik & Yilmaz (2012}) found that consumers do find green ads as a credible and reliable source of environmental information if presented by a credible celebrity endorser.

Clever transmission of green ad messages can encourage the ideals of environment-friendly behavioural intentions among the younger generation like millennials (^{Makgosa, 2010}). Here, it can be implied that green ads play a significant role in integrating sustainable behaviours in social lifestyles (^{Alniacik & Yilmaz, 2012}). It is therefore essential to implement celebrity endorsements in strategising advertisements to attract the attention of both the general public and millennials (^{Pandey, 2011}). A study performed by ^{Aman, Harun & Hussein (2012}) found that celebrity endorsement leads to a more positive impact on the effectiveness of a green ad compared to those not celebrity endorsed. Ads were also found to be more likeable, credible and trustworthy when celebrity-endorsed (^{Karasiewicz & Kowalczuk, 2014}). A significant association was found between endorsement and green ads with the resulting impact of a particular product type (^{Eren-Erdogmus et al., 2016}). The above only further emphasises the importance of implementing celebrity endorsement.

Thus, using celebrities in green ads is a critical decision that may raise advertising effectiveness (^{Chan, Ng & Luk, 2013}). From the perspective discussed by ^{Blasche & Ketelaar (2015}), imagine you are watching a green ad advertised by Mohamed Salah for example and a comparable one by any company, hotel or resort's marketers; would you have followed and be interested in the ad message advertised by Salah, known with his manners towards the society and youth issues, or would have you selected company's recommended ad? From the above, the following is hypothesised,

Hypothesis (1) Green ads are significantly and positively correlated with celebrities' endorsement.

2.4 Green ads and the intention of millennials towards environmental behaviours

According to the Generational Cohort Theory (GCT), people are divided into generations in accordance to their placement in the historical cycle that comprises of certain events which form individual attitudes and behaviours in each cohort (^{Moore, 2012}). The millennial generation is a demographic segment which includes the population that was born between 1982 and 2002 (^{Hanson-Rasmusse & Lauver, 2018}). This generation is a dominant consumer group who are now travelling more for all sorts of tourist activities compared to older generations to gain an authentic travel experience and sometimes take longer vacations (^{Hamed, 2017}).

In accordance to the 'Theory of Planned Behaviour', future desirable behaviours that millennials are anticipated to exhibit are referred to as millennials' behavioural intentions (^{Ajzen, 1991}; ^{Wong & Lai, 2015}). In the context of this current study, the intentions towards environmental behaviours are referred to as the stated likelihood of environment preservation in tourist destinations. Consumer intention, otherwise also known as 'behaviour orientation', plays a fundamental role in marketing strategies (^{Mkik, Khouilid & Aomari,2017}).

Earlier studies found that millennials are mindful about environmental conditions and the encouragement of environmental laws while also having a high environmental concern (^{Mboga, 2017}; ^{Naderi & Steenburg, 2018}). ^{Aman et al. (2012}) described environmental concern as the degree to which people are aware of the environmental problems and endorse efforts to solve them, showing a willingness to participate personally to their solution. Consequently, millennials who are environmentally responsible in their daily lives will give greater importance to the message of green ads (^{Garg, 2015}). With millennials being environmentally-oriented, messages relayed through green ads become increasingly accepted, which subsequently increases the level of their respective environmental orientation (^{Lu, Bock & Joseph, 2013}).

In the context of tourism activities, achieving eco-friendly behaviours in tourist destinations requires continuous information and awareness campaigns covering different environmental issues that direct people to appropriate environmental attitudes (^{Haytko & Matulich, 2008}). ^{Rashid (2009}) stated that green ads are an attempt to reduce the ecological footprint of millennials, decrease environmental harm and achieve a better life in an ecologically responsible manner.

With this, it can be implied that green ads positively encourage the intentions of millennials towards the environment while also guiding them to enjoy holidays in tourist destinations with due respect to environmental resources (^{Chan, 2004}). Environmental awareness in regards to the favourable and unfavourable behaviours in tourist destinations can be increased. In addition, an understanding of their relationship with the biophysical environment along with its challenges in terms of human interactions and their subsequent impact can be fostered (^{Garg, 2015}). A previous study by ^{Kusuma & Handayani (2018}) positively demonstrated that green ads significantly affect environmental attitudes as well as the intentions for green purchasing. This is in agreement with a study by Delafrooz et al. (2004) that showed the encouragement of consumer purchase behaviour towards products that are environmentally-friendly through green ads. Establishing a culture of eco-friendly responsibilities, shifting negative attitudes of individuals in tourist destinations, reducing harmful environmental impacts and improving quality of life can be achieved through green ad campaigns (^{Kardos et al., 2019}). With the above, it is hypothesised that,

Hypothesis (2) Green ads positively impact the intentions of millennials towards environmental behaviours.

2.5 Celebrity endorsement and millennials' behavioural intentions

Millennial travellers are characterised as curious learners, more diverse, more cautious, most educated, more money-conscious, and technologically savvy (^{Veiga, Santos, Águas & Santos, 2017}; ^{Naderi & Steenburg, 2018}). Millennials prefer watching visual content (videos, photos or music) compared to reading a long-written text (^{Kadekova & Holienčinová, 2018}). Similarly, ^{Attallah and El-Mawardy (2018}) noted that the Egyptian millennials are well educated, sociable and high users of social media and smartphone. They also prefer travelling rather than saving money. This young generation is caring about living a healthy lifestyle, enthusiastic about participating in sustainable practices, and concerned about environmental issues (^{Hamed, 2017}). Marketers are very conscious that millennials are the future dominating generation (^{Ketter, 2020}). So, targeting and attracting millennial travellers in advertising campaigns has become an essential matter as they may influence their peers and family decisions (^{Lee, 2010}).

Communicating green messages to millennials can be well reached through the presence of celebrities in ad campaigns (^{Eren-Erdogmus et al., 2016}). ^{Panda & Thakkar (2017}) stated that the star factor of celebrities represents a fundamental core in ads that leads to its popularity and makes them easily remembered and retrieved as they are displayed or purchased. Celebrities play an influential role in the millennial generation (^{Chan, 2011}). Despite millennials not being easily swayed, they show responsiveness as well as a favourable attitude towards all celebrity-involved ads (^{Lotonina, 2019}). Millennials are more probable to be drastically inspired by the meanings and messages endorsed by their favourite celebrity in an advertisement (Chan, 2011).

As found by ^{Yang (2018}), the power of celebrities lies in their ability to influence consumers. For example, scholars found that celebrity endorsement is a success factor in India (^{McCormick, 2016}; ^{Lotonina, 2019}). Most millennials in India are driven by celebrities such as film stars and sports celebrities. They believe more in idealising celebrities and following them since they trust the celebrity. Therefore, they pay close attention to them in ads and have a perception that anything advertised by a celebrity is considered real and superior.

This is also evident in a study by ^{Bush, Martin & Bush (2004}) that demonstrated the relationship between vicarious role models, i.e. entertainers and athletes on the intentions and behaviours regarding consumption exhibited by adolescents in America. In Botswana, a positive relationship was found between celebrity endorsement and purchase intention among teenagers (^{Makgosa, 2010}). The influence of celebrities on the younger generation is emphasised further in a study by ^{Chan et al. (2013}) where the impact of celebrity-endorsed advertisements on brand awareness, perception as well as consumption intention was demonstrated among adolescents in China.

Perhaps the most significant evidence of this issue in Egypt is seen through the advertisement launched by the Ministry of Social Solidarity in collaboration with the Fund for Drug Control and Treatment of Addiction (FDCTA) aimed at inspiring the Egyptian youth to avoid drugs (^{FDCTA, 2019}). The ad has been provided by the Egyptian player, Mohamed Salah, named as one of the TIME's 100 most influential celebrities on April 2019, is looked to by the Egyptian youth as a role model (^{Alrababah, Marble, Mousa & Siegel, 2019}). Salah joined in this ad as a volunteer in the anti-drug campaign under the slogan "You are Stronger than Drugs". The Egyptian musician, Mohamed Hamaki, also participated in this ad through a song that encourages youth to turn any loss into profit and try to live a bestead life. This ad has garnered approximately 32 million views, 80,000 shares and more than 120,000 comments on Facebook, Instagram and Youtube. Moreover, it was found that 88% of the ad's viewers were millennials aged between 18-35 years (^{Aleem, 2017}; ^{Sheleib, 2019}; ^{Wahba, 2019}). From this perspective, driving awareness of millennials towards eco-friendly behaviours can be achieved through the involvement of celebrities in green ads.

However, one of the main challenges of celebrity endorsement in advertising is how to choose the right celebrities to convey the message of the ad (^{Carroll, 2009}). Selecting a suitable celebrity is an essential decision to make and numerous factors such as popularity, reliability, appearance and the harmony between a celebrity and the ad message should be considered (^{Schouten et al., 2020}). Many scholars have attempted to provide some models to guide marketers in finding a suitable celebrity for their respective ad campaign. Among celebrity endorsement theories, Source Credibility theory (which includes trustworthiness and expertise) and Source Attractiveness theory are the most common and approved models when choosing celebrities in ads (^{Ohanian, 1991}; ^{Aureliano-Silva, Lopes, Freire and Silva, 2015}; ^{Cuomo, Tortora, Foroudi & Hussain, 2019}). So, for the current research, three dimensions of celebrity endorsement theories (attractiveness, trustworthiness, and expertise) were chosen from prior literature to assess celebrity attributes that affect millennials.

As for the Source Credibility Theory, the value of the message in a particular ad is determined momentously by the level of expertise and trustworthiness displayed by the celebrity (^{Kumar & Tripathi, 2019}). Trustworthiness is described as the level of honesty, dependability, believability and level of confidence of a celebrity in the endorser (^{Jolly, 2016}). On the other hand, the expertise of celebrities is recognised when people perceive them as qualified, competent and knowledgeable (^{Ohanian, 1991}). The credibility of a celebrity in terms of expertise and trustworthiness are the best incentive for the public to evaluate the ad message conveyed by a celebrity (^{Hani, Marwan & Andre, 2018}). Quite the reverse, the Source Attractiveness Theory assumes that the audience will have a more positive response towards attractive and handsome celebrities. Hence, the success of an advertising campaign is based on how likeable or physically attractive the celebrity is to the consumer (^{Zipporah & Mberia, 2014}). Different results are found in regards to this matter. For example, the study conducted by ^{Eren-Erdogmus et al. (2016}) in Turkey found that highly credible celebrities are more effective in changing attitudes and influencing behaviour. However, Hani et al. (2018) noted that handsome and attractive celebrities transform the ad to an unforgettable experience to the audience. In summary, ^{Cuomo et al. (2019}) stated that it is very important to select the right celebrity from the viewpoint of the targeted audience to boost attention to the message of an ad and achieve the desired goal. Based on the above arguments, the third hypothesis is,

Hypothesis (3) Celebrity endorsement in ads positively affects the intentions of millennials towards environmental behaviours.

2.6 Celebrity endorsement mediates the relationship between green ads and the intentions of millennials towards environmental behaviours

Scholars confirmed that green advertising is one of the methods marketers use to position the environmental values in the minds of consumers as a way to preserve the environment and avoid harming it (^{Kao & Du, 2020}). However, it is not only a matter of spreading knowledge and awareness concerning the environment. Green advertising as a marketing strategy is based on persuading and encouraging consumers to adopt certain behaviours and attitudes that ensure a better quality of life while also refraining from practices that may harm the environment. Therefore, marketers began to commonly use celebrities to play a persuasive role in this regard. For example, the study conducted by ^{Dutta & Singh (2013}) found that celebrity endorsement has a positive impact on encouraging Indian housewives to purchase healthy food brands. Also, ^{Brown (2011}) maintained that organisations select celebrities to support environmental efforts as the celebrity could draw the attention of their entire fanbase to environment-related issues. One of the most crucial roles of a celebrity is to remain in the public eye and mind, and when a celebrity has been chosen to inform the public about environmental issues, the public will certainly listen (^{Brockington, 2009}). Considering the above context, the adoption of celebrity-endorsed green ads in the field of tourism presents a contemporary strategy in maintaining the ecological behavioural patterns of millennials in tourist resorts while also giving them the opportunity to benefit from offered tourism services.

Prior empirical studies which investigated celebrity endorsement in ads ascertained that the message conveyed by a celebrity expressively upsurges advertisement effectiveness and creates a psychological tie with audiences which eventually results in positive outcomes (^{Malik & Sudhakar, 2014}). The same result was found by ^{Aureliano-Silva et al. (2015}) where they have validated that celebrities could affect people's behaviour once they are seen in the advertisement. Also, ^{Pandey (2011}) maintained that ads endorsed by celebrities enhance trusting relationships with individuals which can shape their attitudes through different channels such as TV, Facebook, Instagram, YouTube and many other interactive media networks.

^{Schouten et al. (2020}) asserted that celebrity endorsement in ads has become a widespread way for marketers to support brands, products, and services. Applied to green ads, celebrities could likewise support the preservation of natural and cultural resources in tourist resorts and destinations by appearing in green ads thus attracting their fans and guiding them to the supposed manners towards the environment. Using celebrities and their endorsements of ads has demonstrated itself to be beneficial and cost-effective in the long run despite the strategies required to do so are initially costly (^{Chan et al., 2018}). People look to celebrity-based meanings by using brands and products, practising activities and following recommendations linked with their favourite celebrity to build their own self-concepts (^{Carroll, 2009}). When consumers are attached to a certain celebrity, they have more positive attitudes towards any destination, behaviour, product or brand that the celebrity announces (^{Yen & Teng, 2015}). In the context of green ads, a celebrity can serve as a persuasive tool to drive the intentions of millennials, change their attitude towards the environment, motivate them to have an eco-friendly orientation in tourist destinations and position them as environmentally responsible individuals (^{Jolly, 2016}). This undoubtedly could generate a positive response and acceptance towards messages conveyed by green ads among millennials, especially considering that previous research such as those by ^{Adam & Hussain, (2017}) and ^{Domazet & Kovačević (2018}) has established a strong engagement between celebrities and millennials. They evidenced that millennials follow the behaviours and recommendations supported by their favourite celebrity which will only be beneficial if it is well implemented in green ads.

Consequently, celebrity endorsement in green ads can be developed as a successful communication technique used to fascinate the curiosity of millennials and build credibility with them under the context of "star power". In this regard, ^{Mansour & Diab (2016}) substantiated that celebrities are commonly perceived by the audience as a credible source of information about the ad they endorse and that the message effectiveness will be greater when the credibility of the celebrity is higher. This promotional strategy could greatly assist in improving the awareness of sustainable behaviours that must be practised in tourist destinations among younger generations to guide them in being eco-friendly consumers in relation to the environment (^{D'Souza & Taghian, 2005}). Thus, celebrity endorsement is an important issue for the effectiveness of green advertising whereby the selection of a suitable star can affect an ad's success or failure (^{Amos, Holmes & Strutton, 2008}).

In further justifying the above, additional empirical-based discussions are required by considering celebrity endorsement as a mediating variable between green ads and the intention of millennials towards environmental behaviours. With this, the following is hypothesised,

Hypothesis (4) Celebrity endorsement mediates the relationship between green ads and the intention of millennials towards environmental behaviours.

3.1 The research model

In determining to what extent celebrity-endorsed green ads would influence the intention of Egyptian millennials towards environmental behaviours, the model graphically presented in Figure (1) is proposed.

Figure 1 Conceptual research model 

A quantitative methodology approach was adopted through the usage of a survey questionnaire which is considered to be one of the most suitable research methods that can be used to collect and statistically analyse data, especially in large samples. This method allows the discovery of correlations among different variables within the phenomenon under study, which thus subsequently leads to accurate and reliable results (^{Neuman, 2003}). Collecting data took place between Dec 2019 and Feb 2020.

3.2 Population and sampling procedures

A study population which includes Egyptian millennials aged between 18-38, i.e. born between 1982 and 2002, according to age range adopted by Hanson-Rasmusse & Lauver (2018) is considered in this study. An online survey was placed through Google Drive to collect data. A questionnaire was sent to a random sample of Egyptian millennials via various Facebook groups' sites. A total of 604 responses were obtained from participants. Among them, 580 responses were millennials; aged between 18-38 years old, while the other percentage were eliminated as they above 39 years old. With this, the total number of 580 samples were usable, which thus indicates a satisfactory sample size in accordance with ^{Krejcie & Morgan (1970}). The sample size is also considered acceptable following Kline (2011), who state that 10-20 cases per element in a particular survey is an agreeable standard.

3.3 Construct measurement

A questionnaire with objective questions was developed to collect data. It comprises of six sections drawn from previous literature. With respect to the items and scales used by authors mentioned in Table (2), some modifications were made to suit the research objectives and the targeted sample. After discussions with some experts in the tourism industry, adjustments were made based on their comments through the removal of five items and the rephrasing of some statements to make them more suitable for participants. Likewise, to guarantee the reliability in measuring the empirical data based on statements in the survey and investigate their understandability, a pilot test was carried out with 50 millennials prior to distributing the actual questionnaire. Out of the 50 test surveys, 41 survey responses were valid for analysis. The Cronbach’s alpha for all sections was above the threshold value of 0.7 to be considered accepted. Therefore, following ^{Peterson (1994}), they can be adopted for the current research.

Table 2 Constructs description 

3.4 Data analysis

Data analysis contains descriptive analysis using SPSS and Confirmatory Factor Analysis (CFA) using Amos. A Structural Equation Modelling (SEM) was developed to investigate the research hypotheses. To test this model, a two-phase approach was adapted as recommended by ^{Anderson & Gerbing (1988}). First, model validity and reliability were measured before subsequently testing the goodness of fit for the model. Second, the hypothesised structural model was tested, and a mediation test was performed using the approach proposed by ^{Baron & Kenny (1986}).

4.1. Sample profile

Statistical data shows that 59.3% of millennials are female while males represent the other 40.7%. By studying the background qualifications of correspondents, the highest percentage was for university students (38.6%), followed by those who had a bachelor's degree (27.9%), and high school students (19.7%), while a little percentage of participants either had a doctorate and master degrees (12.8%) or other degrees (1%). Regarding their job, the highest percentage was for students (55.9%), while the remaining percentage was distributed among tourism-related jobs (10%); government employees (11%), private sector employees (10%) and others (10.3%). In view of the number of family members; 52.4% of millennials were in the category of "1-3 persons" followed by the category of "4-7 persons" which represents the remaining 42.8%.

4.2. Concern of millennials in following celebrities

Firstly, millennials were asked if they were interested in following ads presented by celebrities or not. Results declared that 94.5% of Egyptian millennials are interested in ads presented by celebrities. Secondly, regarding the most admired celebrities by young people, Figure (2) demonstrates that the largest proportion of millennials are fans of movie stars (66.9%). Millennials also show a positive concern towards musicians (55.2%) and sports stars (41%) while the least proportion of them follow politicians (11.7%). Thirdly, considering the number of celebrities followed by millennials, responses exemplify that almost half of the sample, i.e. 47.6% follows more than 30 celebrities on social platforms whereas about 40% of them follow between 11 and 30 celebrities. Additionally, by studying the most active social networking platforms used by millennials in Egypt to follow their admired celebrities, findings show that the most preferred social networking platform to follow celebrities were Facebook which represents 76.9%, followed by Instagram (75.3%) and subsequently Youtube (54.5%).

Figure 2 The most preferred celebrities for Egyptian millennials 

4.3 Celebrity traits admired by Egyptian millennials

Millennials were asked to rate the most effective attribute they prefer in celebrities that make them confide in their recommendations and opinions. As shown in Table (3), the major finding in this question was that trustworthiness was the most preferred attribute in celebrity (m 1.722), followed by attractiveness (m 1.912), and lastly expertise (m 2.938).

Table 3 Mean and Standard deviation of celebrity attributes preferred by millennials 

4.4 Measurement model (Testing of the model and hypotheses)

In terms of measuring model validity and reliability, the convergent validity was tested using composite reliability (CR) and average variance extracted (AVE) (^{Fornell & Larcker, 1981}). Initially, statistics of Kurtosis and skewness confirmed the normality of data distribution (^{Bai & Ng, 2005}). Also, as shown in Table (4), the values of CR are ranged between 0.857 and 0.943. Thus, they are therefore considered acceptable as they are above 0.6, which is evident of discriminant validity (^{Scumacker & Lomax, 2010}).

In regards to the measurement of the AVE for discriminate validity, all the constructs show high scores that range between 0.5 and 0.7 which are accepted based on the cut-off point 0.50 recommended by ^{Fornell & Larcker (1981}). The values of Alpha Cronbach (CR) for constructs of the measurement model go above the value of 0.8, supporting the reliability of the measurement model. Consequently, data in Table (4) verifies that the current study has a reliable measurement model.

Table 4 Scale reliability and validity of constructs using CFA 

CFA was performed to assess the validity of the measurement constructs. Findings in Figure (3) support that all the measurement items of the structural model are well loaded in terms of their expected construct. This proves the sufficiency of the measurement items in representing their respective constructs (^{Bagozzi, Yi & Phillips, 1991}). Considering the fitness of the model, the goodness of fit was estimated via model fitting indexes such as the chi-square test (CMIN), which was shown to be significant owing to the large sample size. In addition, the fitness of the model depends on the comparative fit index (CFI) and the Tucker-Lewis index (TLI). It was determined that their values are greater than the recommended cutting point value of 0.9 (Schumacker & Lomax, 2010). It was also determined that the root mean square error of approximation (RMSEA) has a value which is less than 0.07 as per ^{Hair, Black, Babin & Anderson (2009}). With the above, the indices of the structural model (X²= 504.507, X²/df= 3.881, CFI= .939, GFI =.914, TLI= .928, NFI= .920, and RMSEA= .071) provide evidence for a good fitness for the model.

Figure 3 Estimates of the structural model 

Table 5 Estimates of the structural model 

Ns="not significant; **=p<0.05

By analysing the model for the structural equation model shown in Table (5), results revealed that the direct impact of green ads on the perceptions and intentions of millennials towards the environment is not significant (β= .048***, sig= 0.143) thus indicating that green ads are not significant predictors for environment- friendly behavioural intentions for Egyptian millennials. Therefore, hypothesis (1) is not supported.

On the other hand, results show that the path from G.Ads to celebrity endorsement is positive and significant (β= .227***, sig= 0.000) hence implying that green ads are positively correlated with celebrity endorsement. This thus verifies hypothesis (2). Additionally, the hypothesised influence of celebrity endorsement of ads in improving the intentions of millennials towards environmental behaviours as proposed in hypothesis (3) is also accepted as this path is positive and significant (β= .818***, sig= 0.000).

Considering the mediating role of C.END, results obtained from bootstrapping in Table (5) shows that the indirect relationship between G.Ads and MI.EB through a mediator (C.END) is significant. Hence, there is a definite mediating role for celebrity endorsement. Moreover, Table (3) confirms that when C.END enters the model, the direct relationship between G.Ads and MI.EB is insignificant, thus implying that there is a full mediating role for the mediator variable (C.END), since the direct effect is no longer significant after the entry of the aforementioned variable (^{Awang, 2015}). Consequently, hypothesis (4) is supported. With the above, the proposed model in consideration of the standardised path estimates are presented in Figure (4).

Figure 4 Path estimates for research model 

5.1 Discussion

Previous studies have demonstrated the importance of implementing celebrities to convey the message of an ad effectively. The results of the current study corroborate findings of prior research while also emphasising them further by illustrating the power of celebrity endorsement of green ads in encouraging millennials to become more environmentally responsible. Results for the fitness of the structural model considered in this study evidenced that the model is well-fitted. The subsequent sections provide an in-depth discussion for the results found.

Despite research demonstrating the importance of green ads in achieving environment-friendly intentions and sustainable behaviours, this goal was not achieved among millennials in the context of Egypt. The current study has evidenced that green ads without the endorsement of celebrities do not significantly influence the intentions of millennials towards environmental behaviours. This can be attributed to the lack of star power exhibited by celebrities in green ads or the lack of green ads that focus on tourism activities. This is in agreement with a study by ^{Khandelwal & Bajpai (2011}) where no significant linear impact was found between attitudes towards green ads and purchase intention for green products. A low percentage of customers (29.6%) reported the influence of ads regarding eco-friendly products in a study investigating the relationship between customer trust and green ads (^{Hassan & Valenzuela, 2016}). In the same study, only 15% of respondents developed complete trust of the aforementioned ads and the conveyed messages. With green ads being perceived as effective in educating and encouraging youths to go green, the level of green awareness in Malaysia was found to be low (^{Abd-Rahim et al., 2012}). This is in part due to the subpar quality of advertising adhered by the government, which thus undermines their effectiveness. In addition, it may also be due to the lack of celebrities in presenting the aforementioned ads. With the involvement of celebrities in green ads, the messages in the ads can direct the intentions of consumers towards sustainable products, behaviours and practices.

However, in previous studies by ^{Fernando et al. (2016}) and ^{Kim & Yoon (2017}), green ads without celebrities were found to contribute towards an increase in environmental awareness and encouragement in preserving the environment for future generations. Differences in results between studies may be due to the personalities of targeted consumers and the extent they were influenced by celebrities and green ads.

On the other hand, the intention of individuals to engage in particular behaviours was found to be motivated by pressure from peers and community members as well their exposure to green ads (^{Kim & Yoon, 2017}). With this, it can be implied that celebrities can influence millennials and encourage environmentally conscious consumer behaviour without any pressure. In line with this, the presence of a mediating variable in relation to environmental awareness has a positive impact on green consumer behaviour (^{Mkik et al., 2017}). This, therefore, reflects the potential in improving environmental awareness through celebrity endorsement as the mediating variable.

Secondly, findings show a significant correlation between green ads and celebrity endorsement. This is consistent with ^{Blasche & Ketelaar (2015}) in which they argued that in green ads, celebrities could act as a bridge to connect consumers by integrating their image with the message of the ad and disseminating green information to the public to encourage them to go green. Similarly, the research conducted by ^{Brown (2011}) maintained that organisations select celebrities to support environmental efforts as they could draw the attention of their entire fanbase to issues of the environment. According to ^{Panda & Thakkar (2017}), celebrities can notify, convince and influence the general public with specific messages that are being advertised through green ads since the star factor of the advertisement has led to the popularity of the advertisement. With the above, celebrity-endorsed ads can not only promote products, services and destinations, but they can lead to improvements in the attitudes and intentions of millennials towards sustainable lifestyles. Especially when considering the increase in interest to protect and support the environment by organisations all over the world.

Nevertheless, ^{Jolly & Mynavathi (2013}) mentioned in their study that the influence of celebrities on millennials depends initially on whether the millennials themselves are interested in following ads with celebrities or not. They attributed that when millennials are attached to a specific celebrity, they have a more constructive attitude towards the product, behaviour or brand which is backed by the celebrity. This has also been confirmed by other scholars such as ^{Hamed (2017}) who have noted that consumers attached to celebrities perceive the celebrities to be a part of themselves while also having outstanding thoughts and feelings about them. Thus, they may be more willing to focus on the practices and behaviours of celebrities that are shown along with the content in an advertisement. Under this perspective, the results of the current study declared that 94.5% of Egyptian millennials within Egypt are interested in ads presented by celebrities. Furthermore, the most favoured celebrities for Egyptian millennials were determined to be movie stars, musicians and sports stars, respectively. This should be taken into consideration when marketers plan green ads campaigns targeted towards millennials in Egypt. Findings in this study also illuminated that highly trustworthy celebrities are more accepted by millennials to attract and help them recall the message of an ad. This result is in line with ^{Hani et al. (2018}), who pointed out that highly credible endorsers do have an impact on consumer behaviour compared to less credible ones. Yet, ^{Ohanian (1991}) indicated that celebrities selected as endorsers should be experienced and skilled to be truly effective in ad campaigns, thus implying that the celebrity should be an expert. Results have also demonstrated that millennials exhibit a certain level of acceptance based on the attractiveness of celebrities. This is in line with results obtained by ^{Zipporah & Mberia (2014}) that asserted the need for marketers to select celebrities that are highly attractive and would be most pleasant to potential audiences. With the above, Egyptian millennials would therefore develop more positive responses towards trustworthy and attractive celebrities.

Results of this study also reveal that millennials exhibit a significant level of utilising social networking platforms to follow activities, gain information and keep themselves updated about their favourite celebrities. This result confirms the findings found by ^{Moore (2012}) and ^{McCormick (2016}) where they described that millennials are highly interactive media users of social networking websites since it is the most popular channel for them in regards to every aspect.

One of the noteworthy findings in this study is that celebrity endorsement has a positive influence on millennials intentions to be environmentally responsible. It is indicated that millennials are more likely to support ads backed by celebrities and pay attention to their endorsed behaviours as well as recommendations. This result is in line with findings in previous research by ^{Pandey (2011}), ^{McCormick (2016}) and ^{Lotonina (2019}) who confirmed the positive influence of celebrities on millennials' attitudes and perceptions. The younger generation wanted to imitate their respective idols based on their physical appearance, behaviours and attitudes (^{Chan, 2011}). In mainland China, millennial consumers were influenced by celebrities, enjoyed watching their endorsed ads and discussed celebrity news amongst peers (^{Hung, Chan & Tse, 2011}). The same result was also reported by ^{Bush et al. (2004}), who implied that millennials are inspired and influenced by celebrities while also being supportive of the ads they are involved in compared to other unknown individuals.

Hong Kong-based advertisers and marketers would often employ celebrities, i.e. famous athletes and popular movie actors and singers in ads with adolescents as target audiences. ^{Wong & Lai (2015}) has concluded a positive relation between celebrity endorsement with behavioural intentions. With the potential of celebrities to affect the attitudes and behaviours of millennials regarding various issues, they can also, therefore, influence the intentions and perceptions of millennials towards the environment in tourist destinations.

Results in this study have proved that celebrity endorsement (C.END) plays a fully mediating role in the relationship between green ads and the intention of millennials towards environmental behaviours. This is due to the determined significance of the indirect relationship between (G.Ads) and (MI.EB) and the insignificance of the direct relationship between (G.Ads) and (M.BI.E). A non-significant relationship between G.Ads and MI.EB (B=.048, p=.143) was determined in comparing direct and indirect effects. Despite this, the relationship further improved with the entry of the mediator C.END resulting in a significant increase in estimated value to B=0.234 and p=.006. This would therefore imply enhancements for the direct relationship with celebrity endorsement as a mediating factor.

This reflects that celebrities have an attractive power in ads and therefore using them in green ads could make millennials more confident towards the environmental messages conveyed by green ads. Scholars proposed ad campaigns backed by celebrities achieves several benefits such as improvements in advertising effectiveness, the building of trustworthiness, ease of recall in regards to the message of the ad, ad appreciation and the encouragement of positive attitudes towards the tourism environment (^{Mansour & Diab, 2016}). It was evidenced by ^{Brown (2011}) that involving celebrity idols within environmental protection issues certainly reduces negative practices in tourist destinations by motivating millennials to follow their attitude as an example during tourism activities. Ultimately, it can be concluded that celebrities can be a stellar opportunity to encourage favourable behaviours in millennials in regards to the environment during tourism activities.

5.2.Recommendations and practical implications

Based on the above discussions, the current research presents the following recommendations. Firstly, policymakers and marketers in Egypt should pay attention to environment-related advertisements to increase the awareness and create positive attitudes in millennials regarding the importance of preserving the environment and imposing strict penalties on individuals who damage the environment. Secondly, designing and adopting green ads is required from marketers, especially in the context of tourism activities and highlighting the accepted practices in tourism resorts.

In consideration of the current study regarding the interest of Egyptian millennials towards celebrities, effective and successful application of green ads should incorporate the endorsement of celebrities in green ads as a core focus for policymakers and marketers. So, marketers must utilise the opportunity to communicate environmental values to millennials. This approach not only enhances the environmental behaviour of millennials but also generates a positive image of Egypt as an environment-oriented destination. Fourthly, policymakers and marketers in Egypt should focus on targeting millennials in green ads campaigns as they are a considerable generation who travels a lot and play a significant role as a key driver of sustainability among their families and friends. If millennials fully recognise how to act environmentally friendly in tourism resorts and appreciate the importance of preserving environmental resources for themselves, they will recommend others in the future to do the same which would be an effective strategy for promoting sustainable behaviours in tourist destinations. In summary, Figure (5) presents some proposed procedures that should be taken by marketers and policymakers regarding this topic.

Figure (5) Managing the issue of celebrity involvement in green ads 

Afterwards, the current study establishes some practical implications which can serve as a guide for marketers in Egypt by benefitting from the data discussed in the research. It contributes to the existing body of knowledge relating to green advertising and celebrity endorsement. This study provides a starting point for marketers and policymakers to develop green advertising campaigns supported by celebrities to create a better understanding of the accepted practices towards the environment in tourist destinations not only for millennials but also for all individuals in the society. Another implication of this research is to develop a better understanding of how the effectiveness of celebrity endorsement is dependent on selecting a suitable celebrity for the targeted market segment. Trustworthiness was determined to be the most important attribute that must be considered when selecting celebrities for the delivery of green messages to millennials. Also, celebrity attractiveness shows a certain level of acceptance among Egyptian millennials. Consequently, marketers should focus on the credibility and likability of the celebrity for the success of green ads campaigns.

Additionally, the outcomes of this study are useful for marketers in regards to considering the appropriate social networking platforms actively used by millennials to promote green ad campaigns which would enable them to disseminate extensively via word of mouth (WOM) among other millennials. Ultimately, the results of this research will help marketers and policymakers in the tourism sector to acknowledge the significant positive influence of celebrity endorsement on the intention of Egyptian millennials towards the environment. So, they could employ celebrities in various advertising campaigns as a useful tool in helping to solve many different problems in the tourism sector such as increasing cultural awareness in millennials, promoting domestic tourism and encouraging millennials to engage in heritage preservation and sustainable development, especially considering that this young generation frequently travels for many tourism purposes.

Many commercial companies utilise celebrities to promote their products and services. However, using celebrities in ads should not only be limited to promote products and brands. With this, marketers in Egypt need to follow this trend in the field of tourism which would greatly serve the tourism industry in Egypt as it is an important economic sector that includes many environmental, social and economic issues that require continuous support and awareness, especially from influential individuals like celebrities. This study could achieve a positive impact on society influencing public attitudes towards the environment by using celebrities and thus affecting the quality of life.

5.3 Limitations and future research directions

Numerous areas for future research arise from this study. This study has investigated the mediating effect of celebrity endorsement on the relationship between green ads and the intentions for environmental behaviours among millennials. Further research could be extended to make a comparison between millennials and other segments of the market in regards to their response towards celebrities in ads. Other future research should explore the same topic by considering demographic characteristics of millennials as a controlled moderating variable. With this study illustrating that millennials prefer celebrities such as movie stars, sports stars and musicians is an additional interesting study that can be performed to investigate who among local celebrities in Egypt has the most significant influence on millennials.

Furthermore, a noteworthy study could investigate this topic in regards to the level of attachment millennials have with a celebrity. Another comparative study can also be done by considering Egypt and other countries regarding celebrity influence on the behaviours of millennials. Finally, a study on the design of green ads should also be conducted by future researchers.