Collinder 285 - BIg Dipper asterism
Collinder 285 – cluster core stars (image by Niclas Lundin on Unsplash)

Big Dipper is a large asterism – a part of the Ursa Major constellation. Asterism stars do not necessarily belong to the same constellation, and also may not be gravitationally bound. But this is not quite the case for Big Dipper stars, because most of this asterism stars have a common origin. This is what they spectra look like.

Big Dipper spectra - rectified
Big Dipper stars spectra – continuum rectified

As you may immediately notice, the spectra of the six Big Dipper stars are quite similar. Dubhe is an outsider here – it is G9 giant star, that does not fit this company. But if you look closer to these six spectra, you may notice, that Alkaid also differs a little from the other five. It may be better visible at color bar spectra (you may right-click and open image in the new tab):

Big Dipper spectra - color bars
Big Dipper stars spectra – color bars

Alkaid H beta line is a little bit narrower than other stars, the spectrum does not contain so many thin metallic absorption lines, but few more visible helium lines are present. And that is correct – because Alkaid is a hot B3 type star, and the other five are a little bit colder A-type stars.

And that is not a coincidence, that these stars gather in this area. These five stars are the brightest members of the Collinder 285 star cluster – one of the closest clusters. It is also called Ursa Major Stream or Ursa Major Moving Group. A few more fainter stars belong to the cluster core. All these stars were born in one time and place about 300 million years ago. They move in a similar direction, and their composition is also similar – that is visible at spectra. The solar system is located at the edge of this cluster but does not belong to it. Sun is about 15 times older than Collinder’s 285 stars.

Here is Collinder 285 core stars spectra one more time – this time corrected with the instrumental response.

Big Dipper spectra - corrected
Big Dipper spectra – corrected

Clear skies!