# How many times more luminous is the Milky Way compared to our Sun?

## Main Question or Discussion Point

How many times more luminous is the Milky Way compared to our Sun?

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The luminosity of the MW is 10^10 times the luminosity of the sun

10 billion?

EDIT -> 10 minutes late looks like

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Njorl
10000000001 times.

It's like "The Price Is Right" isn't it? Whoever comes closest without going over wins?

Njorl

marcus
Gold Member
Dearly Missed
Originally posted by Njorl
10000000001 times.

It's like "The Price Is Right" isn't it? Whoever comes closest without going over wins?

Njorl
Imamura, an astronomy prof at U Oregon, dares to put a 2 in front and say

http://zebu.uoregon.edu/~imamura/123/lecture-2/lecture-2.html [Broken]

that the luminosity of Milky Way is 2 x 1010 L

that is, 20 billion times the luminosity of the sun. How can he tell the difference between 10 billion and 20 billion? I guess that is what they pay astronomers to do but it seems risky.

However that may be, he has a good webpage on Milky with a list of basic specs like number of stars and average mass of stars and total mass and dark matter etc. Nice and up to date.

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dduardo
Staff Emeritus
\$1 dollar bob.

Greg, this question very subjective. First we all have to agree what region of space defines the Milky way. Then we have to agree on how to measure the luminous. Obviously, as marcus pointed out, astronomers are debating this, so its hard to get an accurate measurement.

I don't even see how we can measure the luminous of the milky way. Do you put the milky way in a big box, create a tiny hole on the surface, and have a device measure this value?

I suspect we are currently calculating it by getting the light we see from other galaxys and subtracting it from the light we see from the milky way, thus cancelling out any background light.

marcus
Gold Member
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Originally posted by dduardo
...astronomers are debating this, so its hard to get an accurate measurement.
1. what keeps the game crisp is that we are really guessing (not about nature itself but) what Greg had in mind when he posed the question. so its clear however it goes. Meteor bets he was thinking 10 billion suns and I am betting he had 20 billion in mind.
either way it goes is fine with me! or whatever other answer

2. I believe they really do have ways. they can, for example measure the mass of a distant galaxy similar to ours and measure its luminosity

and of course they can measure the mass of our galaxy by its rotation curve

so just from that (knowing the mass-luminosity relation for other similar ones) it is not too hard to estimate our galaxy's luminosity, as Imamura and a bunch of other people do

3. and presumably that's not the only way to go about it. Just as a check on the first method, if one has an independent estimate of the proportion of dark matter and one measures the mass of Milky as usual and subtracts the estimated dark matter then one has the mass of the visible stuff mostly stars

and one can look around and see that the average mass of a star is 0.3 solar mass (also an number Imamura gives) so one can estimate the number of stars, and continuing along those lines come up with an estimate of luminosity for the whole

it seems risky, dont try it if your grant depends on it, but heck they probably know what they're doing

4. however what we are really concerned with here is what number Greg had in mind, so we just sit and it will be revealed

I'm going to say 400 thousand million, (2 * 10^11). Why, you may ask, do I go 10 times as high as the previous answer? Well it's because I found this:

"...estimated luminous stars mass only about 175 billion suns."

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marcus
Gold Member
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Originally posted by lavalamp

"...estimated luminous stars mass only about 175 billion suns."
hello lavalamp, while we are waiting for the outcome of the game we may as well converse. I like the "favorite things" song your friend contributed to your website and also the livid lavender pink background color---perhaps I should say colour out of deference.

Indeed 175 billion solar masses is a very good estimate for the mass of the stars in Milkyway galaxy. My source (James Imamura) provides a rough estimate of 120 billion solar masses--and one cannot know these things with fine accuracy.

an interesting thing is if you have, say, 150 billion sunsworth of starstuff and you divide it up into lots of relatively small stars it will glow more dimly and burn more slowly than if you divide it up into fewer but comparatively larger stars.

so to estimate the brightness of our galaxy it is not enough just to say 120 billion or 150 billion or 175 billion solar masses (which sounds like the right range of amounts) but you also have to say what the average size of star is! I gather this is about 0.3 solar mass (anyway down there near half-a-sun in mass)

actually Im pretty sure that Meteor has the right order of magnitude---tho Greg is the final arbiter of course. In other words if you divide 120-175 billion solar masses of star stuff in stars averaging 0.3 solar mass----so there are some 400 billion stars---then you would get a glow of about 20 billion times the sun's luminosity. Because the stuff would be burning on average rather cooler and slower than it does in the sun

dduardo
Staff Emeritus
"How many times more luminous is the Milky Way compared to our Sun?"

luminous can mean intellgent, but both the milky way and the sun are stupid.

marcus
Gold Member
Dearly Missed
Originally posted by dduardo
"How many times more luminous is the Milky Way compared to our Sun?"