sombrero galaxy...?

Question

ok. so, it's 28 million light years from earth. it is 50,000 lightyears across. it has 800 billion suns...
i (sort of) understand how they measured how far away it is but...
how did they measure how far across it is?
how did they count all those suns?

and so i can tell who actually knows from guessers:
how fast is the universe expanding? (km/sec/megaparsec)?

i'm NOT a doubter. just a person whose carreer does not stimulate me.

braxton_paul,
thanks, very informitive.
"measuring the angle it subtended and doing the trig?"??-- what does this mean? it's over my head. 1 or 2 small paragraphs to explain this would be appreciated. thanx :)

Answer 1

Expansion rate of the universe is about 70 km/sec per mega parsec.

"...how did they measure how far across it is?..."
Once the distance to the Sombrero was determined, then it was a matter of measuring the angle it subtended and doing the trig.

"...how did they count all those suns?..."
No count was done. Intead the assumption of the number of stars required to generate the observed luminosity gives a fairly accurate stellar count.

Answer 2

I am more of a sceptic. I read Brecht;s poem, "In Praise of Doubt" at an early age and it has influenced my thinking.

There has to be a margin of error in any estimation. As regards the number of stars in the Milky Way for example, whilst it was thought to be 200 billion, the latest view as reported in Wikipedia is 200-400 billion depending on the numbers of smaller stars (78% of Main Sequence stars are low-mass red dwarfs so I would have thought that this was known. Certainly the percentage of the nearest stars that are red dwarfs is consistent with that.)

And a further complication is the amount of dark matter. It seems you can't just divide the mass estimated from the total luminosity by an average mass per star, any more.

Most of the mass of the Milky Way is thought to be dark matter, forming a dark matter halo of an estimated 600-3000 billion solar masses which is concentrated towards the Galactic Center.

So I would raise a querying Brechtian eyebrow at the figure of 800 billion stars in the Sombrero Galaxy and say "if we are so uncertain about the numbers in our own galaxy, how can we be so sure that this estimate is any more "exact" than that?"

In addition there are some unusual features of the Sombrero Galaxy that complicate things.

(1) the massive dust lane around the central bulge, which is thought to be the principal area of new star formation

(2) the relatively high number of star clusters (2000+) of hot young stars (the Pleiades is our local example)

(3) In the 1990's, a research group led by John Kormendy demonstrated that a supermassive black hole is present within the Sombrero Galaxy.

Using spectroscopy data from both the CFHT and the Hubble Space Telescope, the group showed that the speed of rotation of the stars within the center of the galaxy could not be maintained unless a mass 1 billion times the mass of the Sun, is present in the center. This is among the most massive black holes measured in any nearby galaxies.

These features would all seem, to me to skew calculations. It is not an average kind of a galaxy.

=

Fortuitously, this question has attracted 3 of the top 4 contributors to Astronomy and Space category and all in a row, too Campbelp is way out in front in 1st place, Paul Braxton is 3rd and I am breathing down his neck in 4th place. Curious that Yahoo don't consider Paul to be worthy of an orange gong by his name. If he is the 3rd most prolific answerer as measured by number of best answers, does that not merit the words "Top Contributor" by his name? If it doesn't, what does? Seems to me this devalues the gong somewhat.

Answer 3

I don't know the rate of universe expansion off hand

however

Two ways of measuring the distance:
1. Red shift. Measure the difference in emission lines between known elements (hydrogen) and those observed. The difference is a kind of Doppler shift which tells you how fast something is moving away from you. Using Hubble's law, you can calculate the distance to it.

2. A Cepheid variable or Cepheid is a member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity. Since we can then get an accurate measure of absolute luminosity, we can calculate distance quite accurately. The Sombrero galaxy is close enough that some individual Cepheids have been identified.

Once you know the distance to something, and you know it's apparent diameter, you can use straight forward trigonometry to calculate it's actual distance.

The don't "count" the suns. This is an estimate based on total luminosity.

Hope this helps.

Answer 4

Sombrero Galaxy

The Sombrero Galaxy (also known as M104 or NGC 4594) is an unbarred spiral galaxy in the constellation Virgo. It has a bright nucleus, an unusually large central bulge, and a prominent dust lane in its inclined disk. The dark dust lane and the bulge give this galaxy the appearance of a sombrero. The galaxy has an apparent magnitude of 9.0, making it a galaxy that can easily be seen with amateur telescopes. The large bulge, the central supermassive black hole, and the dust lane all attract the attention of professional astronomers.

http://www.answers.com/Sombrero%20galaxy

Answer 5

I was going to answer but braxton_paul got it 100% correct, so just refer to his answer.

Answer 6

i duno/???