Re: Are the stars we see at night still there?

7/11/00

Dear Jim,

You're absolutely right; especially with regard to faraway galaxies, for which the light that we see them by was made in massive young stars whose lifetimes are much shorter than the time taken by their light to reach us. Most of those stars are *not* still there, and we can never know what they look like "now"!

That sounds depressing, but look at it this way-- seeing galaxies of stars as they were some time ago proportional to their distance gives us astronomers a vantage point on our universe that historians of human endeavors must envy. To be able to merely *look* directly into the past.

Stars live stable lifetimes (on the main sequence, thermostatically burning hydrogen into helium) that are proportional to the ratio (mass/luminosity). It makes sense-- divide total energy stored by the rate at which you use it up, and you get the time before you've used up your energy.

Because the stars have to liberate enough energy through fusion to balance their weight, more massive stars burn their energy a lot faster than smaller ones. Under more pressure, the most massive stars rip through their core hydrogen supply in only a tiny fraction of the time that it takes for the smallest stars.

For the Sun (on the smallish end of things, but interesting nonetheless), the stable lifetime is about 10 billion years (which we're about halfway through now). So, you know that if you see Solar mass stars more than about 10 billion light years away, they probably don't exist anymore. But that's pretty far away-- at a redshift of about 0.8 for a Hubble constant = 75 km/s/Mpc, and 10^10(ltyr)/3.26(pc/ltyr)= 3000 Mpc. It's out in the superclusters of galaxies!

But--the most massive stars are about 40 times the mass of the Sun. So they put out a lot more power, and die a lot quicker; in less than a couple of million years only. Well, a couple of million light years isn't very far now. It's only about 600 kpc-- just about the distance to the nearest galaxy, M31 (Andromeda).

The most massive stars are the most luminous, so those are what we tend to see when we look at galaxies other than our own. Bright lights are easier to see from far away than dim ones, right? Our quick calculations have shown us that even when we look at the nearest galaxies to our own, we are likely seeing them very differently from how they would look "now" in their own reference frame. I hope that answers your question. I'm sorry it took so long to reply, but I was out sick for a few days.

Thanks,

Kristin Nelson-Patel :)