# The Naked Scientists Forum

### Author Topic: Mass density distribution of galaxy  (Read 5977 times)

#### Maze

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##### Mass density distribution of galaxy
« on: 10/03/2009 10:36:36 »
What is the approximate mass-density distribution within a galaxy, i.e. what is the plot of the visible Mass-density vs Distance from centre of the galaxy?

I am not interested in the dark matter component, only the visible or baryonic matter.

Clearly the mass-density will be greatest at the centre of the galaxy and drop off exponentially with distance from the centre.
This drop off may vary across galaxies and might be difficult to determine because of the presence of dark matter but I am only after an approximation.

Maybe the mass-density vs dist. is proportional to the 1/(dist^2) or maybe 1/(dist^3) etc.

#### DoctorBeaver

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##### Mass density distribution of galaxy
« Reply #1 on: 10/03/2009 10:44:36 »
Which type of galaxy are you referring to? There are different kinds.

#### Vern

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##### Mass density distribution of galaxy
« Reply #2 on: 10/03/2009 14:03:59 »
Most sources have the Milky Way Galaxy visible mass at 200 to 600 billion solar masses. It is approximately 100,000 light years in diameter, and it exists approximately as a circular disk about 20 light years thick. If you're good at arithmetic just solve for the area and divide by the number of solar  masses.

Here is a typical site that turns up in a Google search. The consensus seems to be in the range of 200 to 600 solar masses for our Milky Way galaxy. Other galaxies may be much larger or much smaller.
As we know, a galaxy is a huge collection of millions, billions, or trillions of stars. The galaxy we live in is called the Milky Way. The name comes from the literal translation of the Latin "Via Galactica", with via meaning "road"or "way"and the word galaxy from the Greek root "gala", meaning "milk". The Milky Way galaxy is composed of about 400 billion stars and is about 100,000 light years in diameter.

The Milky Way is classified as a spiral galaxy and is composed of three main regions:

1. the disk, where the solar system is located,
2. the central bulge at the core, which is densely packed with stars and presumed to have a black hole at its center, and
3. a halo,a diffuse region with a low density of stars, that surrounds everything. The halo is believed to be composed mainly of dark matter that exceeds even beyond its ends.

The total mass of the Milky Way is assumed to be at least 600 billion times the mass of the sun, while the densely packed visible part is only 200 billion times the mass of the sun. This discrepancy in numbers is believed to be caused by the dark matter in the halo, since it seems to be taking up mass, but doesn't emit or radiate light. This "missing mass"accounts for almost 90% of the mass in the universe. Even though scientists don't know what it is, they know it's there because they can detect it by the gravitational effect it has on the surrounding visible objects.

Unfortunately, this unknown dark matter is also the determining factor in the evolutionary future of the universe. If there is too little of it to gravitationally bind the the universe together, it can continue expanding forever. If there is enough, though, the universe might slow down the expansion, come to a halt, and begin to contract and eventually collapse. This is why it is so important to find out just what dark matter is and how much of it there is.

Alina Vayntrub -- 2000
« Last Edit: 10/03/2009 14:27:21 by Vern »

#### Vern

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##### Mass density distribution of galaxy
« Reply #3 on: 10/03/2009 14:11:35 »
If someone actually gets out their calculator, there is a puzzle that I have often thought about. Stars live about 10 billion years; we can assume that the galaxy is at least that old. Every star spews out a solar wind of ions and light of all frequencies. All this stuff goes out from the galaxy. Some of it probably finds a stable orbit.

So how much does 10 billion years worth of star dust weight. Might this be a candidate for the missing matter in galaxies? At first it seems that there could not possibly be enough mass; but ten billion years?

#### Maze

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##### Mass density distribution of galaxy
« Reply #4 on: 11/03/2009 10:41:20 »
DoctorBeaver
I would be interested in the density profile of any type of galaxy if such information is available.

Vern, thanks for the data but it would only give me the overall density of the Milky Way. I am hoping to find out an approximation of how the density varies with distance from the centre of the galaxy.

#### Vern

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##### Mass density distribution of galaxy
« Reply #5 on: 11/03/2009 11:06:06 »
Maze
That would be a very good study and might lead to some universal principal that could be applied to understand a more general distribution of mass throughout the universe. However, I don't know of such a study.

#### Soul Surfer

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##### Mass density distribution of galaxy
« Reply #6 on: 11/03/2009 23:28:32 »
Stars in general do not throw of a significant amount of their mass until they reach a late stage in their life cycle.

The life cycle of stars varies very greatly from a few million years for very high mass stars through about ten billion years for a sun type star up to thousands of billions of years for small red dwarfs

stars of around solar mass and below throw off their mass gently and become planetary nebulae to for white dwarfs which just take a long time cooling down.  Larger stars do their mass shedding more violently forming recurrent novae or supernovae.

The material shed by stars cools down to form more stars and a lot has probably been recycled several times by now because of the short lives of the larger stars.

#### Kmilin

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##### Mass density distribution of galaxy
« Reply #7 on: 27/07/2009 02:02:34 »
Hi Maze, i hope this helps, it was copied from Binney Tremaine, page 111,

(a) The Bulge the density of this components is assumed to be

ρb(R,z)=ρb0(m/ab)^(-alphab) exp{m^2/rb^2}

where

m = sqrt{R^2 + z^2/qb^2}

For qb < 1 this is an oblate, spheroidal power-law model that is truncated

Near-infrared photometry (BM x10.2.1) suggests values for three of the parameters,
alphab = 1.8, qb = 0.6, rb = 1.9 kpc, and without loss of generality, we can set ab = 1 kpc. The parameter ρb0, and hence the mass of the bulge, are determined by tting the dynamical constraints.

...

(c) The stellar disk The density of the stellar disk is assumed to fall o
exponentially with radius R, as in equation (1.7), and to depend on distance
from the midplane z through the sum of two exponentials, representing the
thin and thick disks described on page 13|this dependence on z is motivated
by observations such as those of Gilmore & Reid (1983), shown in BM Figure
10.25. Mathematically,

ρd(R, z) = Σd exp{-R/Rd}((α0/2z0) exp{-|z|/z0}+(α1/2z1) exp{-|z|/z1})

where α 0 + α 1 = 1, ∑d is the central surface density, Rd is the disk scale
length, and z0 = 0.3 kpc and z1 = 1 kpc are scale heights for the thin and
thick components.

sorry i cant help more than that but i was looking for the same stuff, its a shame no one uploaded a plot -observed- or at least smthn about the density profile  [V] [V] [V]

#### Kmilin

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##### Mass density distribution of galaxy
« Reply #8 on: 27/07/2009 02:04:10 »
...for the milky way

#### The Naked Scientists Forum

##### Mass density distribution of galaxy
« Reply #8 on: 27/07/2009 02:04:10 »