Extra-galactic view

[DrYool]

Hi everyone,

Today's APOD shows a "nearby" globular cluster with what's quite a small number of stars in it - less than half the number of people in the city in which I live in. It made me wonder what sort of night views a planet might have when there are only tens of thousands of local stars instead of the millions that we're used to. And then I got to wondering what a night view would be like for a planet (or spaceship) in extra-galactic space. Presumably it would just be of distant galaxies, but just how visible would these be? Anyway, in this vein, I was wondering if anyone had ever tried to reconstruct a sky view with local (i.e. Milky Way) stars removed. I don't imagine that it would be an easy thing to do, but I also couldn't quite imagine what it would actually look like. Any ideas? Anyone ever tried this? I guess that Google might have thought about it.

Cheers,

Andrew.

[Markus Schwarz]

It made me wonder what sort of night views a planet might have when there are only tens of thousands of local stars instead of the millions that we're used to.

This gives an artistic answer to the inverse of your question: what might be the night view of a planet inside a star cluster.

If one were to remove all Milky Way objects from our night sky, it would be pretty empty. Only the moon and the planets would remain. In the southern hemisphere, you could see Andromeda and the Magelanic glouds, but that's about it. AFAIK, they are the only galaxies that you can see with your own eyes. All in all, I believe such a sky would be extremely boring.

I was wondering if anyone had ever tried to reconstruct a sky view with local (i.e. Milky Way) stars removed.

However, given a sophisticated telescope, such as Hubble, and staring at a tiny "empty" portion of the night sky long enough, you get the Hubble Deep Field image.

[Chris Peterson]

Today's APOD shows a "nearby" globular cluster with what's quite a small number of stars in it - less than half the number of people in the city in which I live in. It made me wonder what sort of night views a planet might have when there are only tens of thousands of local stars instead of the millions that we're used to.

The star density in that little cluster is a LOT higher than it is in our vicinity. With our eyes, we don't see millions of stars in the sky, only a few thousand (count them... you can). If you were inside this cluster, you'd see nearly 100,000 stars visually- and many would be very bright. So the nights would be very starry, and a few of those stars would be daytime objects, as well.

And then I got to wondering what a night view would be like for a planet (or spaceship) in extra-galactic space. Presumably it would just be of distant galaxies, but just how visible would these be?

You'd see no stars (or maybe the rare one, depending on how many extragalactic stars are out there... not many, that's for sure), but you would see galaxies. They'd look like the naked eye galaxies we can see from Earth- small, dim, fuzzy gray spots of light.

[Sam]

A few of those stars would be daytime objects, as well.

Maybe not strictly on topic, but I've been wondering for a while what a daytime star would look like--none of our daytime sky's visible objects (Sun, Moon, Venus, Jupiter) are point sources, so I have sometimes thought that that's the reason for their visibility.

On the other hand, the night sky has a brightness too, so I guess it's a difference of scale not of type. If a star is brighter than the sky in front of it (which varies with distance from the sun and horizon) it will be visible. A test of this would be to calculate when the sky around, say, Sirius reaches the -1.46 magnitude of that star and compare with when the star actually becomes visible.

Am I thinking about this correctly?

--
Sam

[Chris Peterson]

Maybe not strictly on topic, but I've been wondering for a while what a daytime star would look like--none of our daytime sky's visible objects (Sun, Moon, Venus, Jupiter) are point sources, so I have sometimes thought that that's the reason for their visibility.

Visually, Venus and Jupiter are indistinguishable from point sources. These objects only approach our visual resolution, and are usually much smaller. So their appearance at night is comparable to bright stars, and during the day to daytime stars. Venus, of course, is not too difficult to see in the daytime, while Jupiter is challenging.

Venus is magnitude -4 at its brightest; inside a cluster, many stars would be magnitude -8 or brighter- 40 or 50 times brighter than Venus. Those would be easy to see. Iridium satellites can flare at -8 magnitude, and are predictable. Watch for one that will occur during the day at your location, and you can see what mag -8 really looks like.

On the other hand, the night sky has a brightness too, so I guess it's a difference of scale not of type. If a star is brighter than the sky in front of it (which varies with distance from the sun and horizon) it will be visible. A test of this would be to calculate when the sky around, say, Sirius reaches the -1.46 magnitude of that star and compare with when the star actually becomes visible.

You need to consider the optics involved. With a telescope, many stars are visible in the day. Sirius is easily seen with binoculars, although I've never managed to see it with just my eyes.

[Sam]

Watch for one that will occur during the day at your location, and you can see what mag -8 really looks like
...Sirius is easily seen with binoculars, although I've never managed to see it with just my eyes.

Oh, cool--I'm going to try this!

Thanks!

--
Sam

[DrYool]

Dear all,

Thanks for all of your comprehensive replies. Of which, it sounds like I should be grateful to be living within a galaxy populated sufficiently with stars for the night sky to be interesting! Though being in a cluster with neighbouring stars close enough to be visible during daytime would be pretty cool. But, then, would night-time seem quite so special?

Thanks again.

Andrew.

[BMAONE23]

Depending on yout location within the cluster, you might have over 1000 stars with a mag brighter than Sirius and likely brighter than Venus. Since the orbital dynamics in a globular cluster are different, your stars also wouldn't have fixed positions forming constellations. The brightest would appear to be constantly changing positions in the sky. Some of the less bright ones might be more stable over the course of decades and lifetimes though.