Size of Local Milky Way Arm

Kitsune · Post by **Kitsune** » 2005-04-11 11:02pm

Has anyone read any articles with estimates on how wide our local Milky Way arm is and how think or tall it is?

The Spartan · Post by **The Spartan** » 2005-04-11 11:12pm

If I remember right, our arm is 3000 Light-Years wide.

Shinova · Post by **Shinova** » 2005-04-11 11:26pm

I have always thought that our galaxy was something like 5 or 500 million light years wide.

Now our galaxy seems so....... tiny.

The Spartan · Post by **The Spartan** » 2005-04-11 11:41pm

Shinova wrote:I have always thought that our galaxy was something like 5 or 500 million light years wide.

Now our galaxy seems so....... tiny.

Our galaxy has a diameter of about 100000 Light-Years. But the thickness in our neighborhood, so to speak is 3000 Light-Years. For comparison the bulge in the middle of the galaxy is 16000 Light-Years thick.

Shinova · Post by **Shinova** » 2005-04-11 11:47pm

Whew, 100,000 is a lot better than 3,000. I was beginning to feel claustrophobic.

Grandmaster Jogurt · Post by **Grandmaster Jogurt** » 2005-04-12 02:21am

The Spartan wrote:Our galaxy has a diameter of about 100000 Light-Years. But the thickness in our neighborhood, so to speak is 3000 Light-Years. For comparison the bulge in the middle of the galaxy is 16000 Light-Years thick.

You got that from the Monty Python song, didn't you?

Another point of interest is that we're approximately 30,000 lightyears from the center, if that song is correct.

Lord Zentei · Post by **Lord Zentei** » 2005-04-12 06:26pm

No. No, no, no.

* The galactic disk is 40-50 kpc across, which places it in the range of 130000-160000 light years. It's number density follows an exponential decay law.

* The stellar halo is 100 kpc across. It's number density follows a polynomial decay law, 1/r^3.5 moving outwards from the galactic center.

* The galactocentric distance of the Earth is 8.0 +/- 0.5 kpc.

* The scale length of the disk is 3.5 kpc.

* The scale height of the thin disk is 325 pc.

* The scale height of the thick disk is 1400 pc. It's number density at the galactic midplane is only 2% of that of the thin disk.

* The diameter of the central bulge is 2 kpc.

* The spiral arms are density waves: the dense regions are ca. 20% more dense than the regions between the arms at the same galactocentric distance.

Lord Zentei · Post by **Lord Zentei** » 2005-04-13 11:37am

In case there is anyone confused about what I was saying above: scale length/scale height means the distance over which the number density drops by a factor of e=2.71828.

The disc is composed of the "thin disk" and the "thick disk". The stellar distribution along the "vertical" axis of the disk requires a sum of two exponential decay functions in order to explain it. The young thin disk, of which the Sun is a member, has a scale height of only 50 pc and is associated with stellar formation; the thick disk consists of old stars. The Sun is located 30 pc "above" the galactic midplane.

The stellar halo consists of an estimated 230 tightly packed globular clusters (only over a hundred actually known, others are inferred) and so called field stars which are distributed very thinly between these clusters. Of the halo stars, only 1% lie in the globulars. Most stars in the halo are ancient, though globular clusters seem able to pick up young stars as they sweep through the disk in their orbit around the galactic center.