I always wondered about this concept of showing the Arundhati nakshatram during hindu marriages especially in the south. I never understood the significance of the custom which is followed without fail, until today. Read the following article which clarified it. Nice to know, there are people who know the significance and kind enough to share it.
We look up to virtuous people as role models. But, sometimes it happens that those who are virtuous are proud of their good conduct and proud of their devotion to God too. When they display such pride, God does not fail to take them down a peg or two. This was seen in the case of the Saptarishis too, Sarala Rajagopalan said, explaining the significance of the star Arundhati.
The story goes that the Saptarishis became arrogant because they felt that no one could perform penance better than they. Their wives had the idea that there were no women more virtuous than they, and there could be no wives so loyal as they were. Such pride would be inexcusable in anyone, and for sages and their wives to have such pride was unthinkable. So Lord Siva decided to teach them a lesson. So He took the form of Bikshandar. Lord Narayana joined Siva by taking the form of a beautiful woman Mohini.
Since the sages claimed that they were steadfast in their devotion to God and they had no thoughts except thoughts of the Divine, Lord Narayana, as Mohini, decided to test their will power. So He went to the place where the sages were involved in worship. The sages took a look at Mohini's beauty and forgot themselves, and about their penance. They forgot that they should be beacon lights to others through their own upright conduct. They were taken in by Mohini's beauty!
As for their wives, the moment Siva as Bikshandar made His appearance before them, they too began to admire His good looks, forgetting that they had not so long ago claimed that they were among the most virtuous in the land! But among the sages, Vasistha was not tempted by Mohini. And among the women, Arundhati was not tempted by Bikshandar. That is why Arundhati was granted an exalted position. She was made part of the Saptarishi mandalam (the Great Bear constellation), and is seen next to Vasistha in the constellation. At the time of marriage, the bride is asked to see the Arundhati to indicate to her that she should at all times be loyal to her husband. The presence of Arundhati near her husband Vasistha also shows that a husband and wife must always remain united.