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Sirius B was a red giant once


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I recently read that, until about 120 million years ago, Sirius B, the Pup, was a red giant star until it collapsed into its current form as a white dwarf.

That got me thinking. 120 million years isn't a long time in astronomical or geological time. Life was abundant on planet earth. I just wonder what a red giant star only 8 light years away would have looked like to our animal ancestors.

Currently, the closest red giant is apparently Gamma Crucis at magnitude 1.63 at a distance of about 90 light years. Presumably, Sirius B would have been many, many times brighter being ten times closer. Any ideas what its magnitude might have been.

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Hi Luke,

           You have to remember that  the Sun and Sirius are on completely separate orbits around the Milky Way`s centre. 120 million years ago they could have been on opposite sides of the Galaxy.

According to Fred Schaff`s book " The Brightest Stars " Sirius only became visible about two million years ago and will remain visible for about the same time in the future.

If our paths had crossed when the Pup was in it`s red giant phase it would certainly had shined brighter than Venus and would have cast shadows.

Steve.  

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I agree with Steve, the current close proximity of Sirius is a relatively short event. Here's a graphic showing how proper motion of stars change their distance to us over tens of thousands of years (and not millions!)

Before the red giant phase, Sirius B was a bright B type star. 240 million years ago the Sirius system wasn't even born yet. B stars have a very short life span.

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1 hour ago, Waddensky said:

I agree with Steve, the current close proximity of Sirius is a relatively short event. Here's a graphic showing how proper motion of stars change their distance to us over tens of thousands of years (and not millions!)

Before the red giant phase, Sirius B was a bright B type star. 240 million years ago the Sirius system wasn't even born yet. B stars have a very short life span.

thats a great article its amazing weve lasted so long

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On 09/09/2020 at 18:43, lukebl said:

Currently, the closest red giant is apparently Gamma Crucis at magnitude 1.63 at a distance of about 90 light years. Presumably, Sirius B would have been many, many times brighter being ten times closer. Any ideas what its magnitude might have been.

Arcturus is a red giant "only" 36 light years way. 

Sirius B is currently eight light years away. Suppose it was still eight light years away 120 million years ago (it wasn't as has been explained, but just suppose).  So Arcturus is a tad more than four times further away.  So since light intensity decreases as a square of the distance, since it is four times further away and assuming they were both of the same absolute magnitude (maybe Sirius B was much brighter than Arcturus) means Sirius B would have been 4 squared or 16 times brighter than Arcturus is today, again assuming the same absolute magnitude.  In other words it would have been a very, very bright, red star at about magnitude -4 or -5 if my reasoning is correct, a red version of how Venus appears to us, roughly. Maybe a bit brighter.

So it would be very, very bright, certainly, but nothing unbelievably spectacular - 8 light years is still a vast distance.  Now, if it had blown as a supernova we'd have been in trouble!

Edited by kirkster501
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