Galaxies-I

By the 1700’s the old notion that the Earth was the center of the Universe was overthrown by the success of Newton’s theory of universal gravitation, a theorywhich explained the motion of the planets around theSun.

It became respectable to see the stars as other suns, like ourSun, but scattered through space, with the possibility of other planets around those suns and even intelligent life on those planets.

And so the question arose: where is our Sun in this universe of stars?

Answering this question required 200 years, roughly from 1750to 1950. To answer it, astronomers started from carefulobservations of the distribution of stars and star clusters in the night sky.

The Milky Way is a hazyband of light visible to the naked eye in the evening sky.In a telescope it resolves intoclouds of stars.

In 1750 Thomas Wright suggestedthat the existence of the Milky Wayproves that the stars are notrandomly distributed, but distributedin a flattened disk with the Sun offto one side.

In 1785 William Herschel countedstars in 683 regions of the sky andused these counts to estimate the sizeof the disk and the direction to the center.

horizon

Milky Way

Scutum star cloud

Sagittarius star clouds

Herschel’s picture of the system of stars (1785).

Herschel’s estimated position of the Sun.

Illustration by Wright showing stars confined to a plane (1750).

Direction ofSagittarius star clouds

The system of stars which containsour Sun is now called the Galaxy

•A composite of 77 photographs showing details of the star clouds in the MilkyWay. By the early 1900’s the dark areas were understood to be obscuring cloudsof dust and gas. This source of interstellar absorption invalidated the star counts Herschel (and later astronomers) used to estimate the size of the Galaxy.

This is a more recent color mosaic of the Milky Way created by an amateur astronomer.

•Open clusters are atype of star cluster thattypically contain 100 to1000 stars.They are found wherestars are forming out ofthe interstellar medium.

•The Pleiades in Taurus

(photo from Bok, 1976)

•The distribution of star clusters helps us understand the structureof our galaxy.

The Milky Way in Cygnus.Open clusters, yellow circles onthis chart, are concentrated along the Milky Way. Gaseous nebulae, in green, are also found concentrated along the Milky Way.

“Veil nebula” supernovae remnant

NorthAmerican nebula

An example of a gaseous nebula is the “North American nebula,”

NGC 7000, near Deneb in Cygnus. Note the evidence of dark clouds by comparing star counts in the region on the right to the region on the left.

Star clusters condenseout of the dust and gasbetween the stars.

HII regions are gaseousemission nebula glowing from the ultraviolet light from hot young stars, the red coloris distinctive

Globular clusters, in contrast to open clusters, are concentratedin one part of the Milky Way. This picture of Sagittarius star clouds contains one-third of all known globulars in an area less than 3% of the sky.

•A typical globular clusterhas 500,000 stars.

The distribution of open clusters in thesky as seen fromEarth, in a coordinatesystem aligned withthe Milky Way. The center (marked by a cross) is in Sagittarius. The horizontal lineis the plane of the MilkyWay.

The distribution ofthe globular clusters in the sky. They are strongly concentratedin the direction of Sagittarius.

NorthGalacticPole

SouthGalacticPole

NorthGalacticPole

SouthGalacticPole

Harlow Shapleyproposed, circa 1918-1921, that the center of the distribution of the globular clusterswas the true centerof the Galaxy.Thiswas slowly accepted.

The position of the Sunis marked S in this diagram.The position of the centerof the Galaxy in Shapley’smodel is marked C. Thegray area around S is aboutthe size of Herschel’s modelof the Galaxy from star counts.

(Trumpler, 1930)

Magellanic Clouds

Though Shapley’sdistances were overestimated,his idea was correct.By the 1950’s the overallsize and the shape of the Galaxy was thought to be well understood.

The Galaxy consists ofa disk population, Population I, of relatively young stars and a halo population, Population II, ofolder stars, remnants of the formation of the Galaxy. The central bulge also contains older stars. Two small satellite galaxies, the Large and Small Magellanic Clouds, slowly orbit our Galaxy.

Disk

HaloCentral Bulge

The Disk:The open clusters are Population I and found in the disk of the galaxy. The distribution of HII regions, open clusters, and dark molecular clouds where stars are forming is not uniform in the disk. Theseobjects trace out spiral arms. The best measurements of the spiralarms are made by observing the 21-cm radio emission of neutral hydrogen gas. Neutral hydrogen is concentrated in the spiral arms.

21-cm radiation is emitted when the spin of the electron in thehydrogen atom changes from spin-up to spin-down. The hydrogenemits a photon with an energy equal to the energy difference of the two states:

Higher energy: spin-up Lower energy: spin-down

This is a radiomap of the 21-cm radiation from the diskof our galaxy.

The dark areas of strong21-cm emissionoutline the spiral arms.Maps like thisonly becamepossible afterWWII.

IR view of the Milky Way (2MASS) showing central bulge.

Inside the bulge is thecenter ofthe galaxy.

We will studythis in moredetail later.

An artist’s conception of the Milky Way Galaxy seen face-on. This is based on radio maps and other sources of data. The yellow dot is the position of the Sun: 10 kpc from the center of the Galaxy.

30 kpc

From the motions of stars in the galaxy, and the 21-cm data, we knowthat the galaxy rotates. The Sun takes ~250 million years to completeone orbit around the galactic center. The inner stars complete theirorbits in shorter times. This differential rotation would destroy thespiral arms though, winding them up in a few hundred million years.

If the galaxy is billions of years old, why do the spiral arms persist?

Density wavetheory is one solution.

Thedensitywave, likea waterwave, movesthroughthe diskcausingstars to form by compressing the gas anddust in the disk, but hardly affecting the stars at all.

Another theory is self-propagatingstar formation, where “waves” ofstar formation can form bands ofyoung stars in the galactic disk.

1) Cluster of stars form out of part of an interstellar cloud.

2) The hottest stars ionize the gassurrounding gas and an HIIregion forms. The HII regioncompresses the cloud nearby,causing new star formation.

3) The most massive stars become supernovae and the expanding shock waves also cause star formation.

4) Differential rotation then shears these regions into arcs we see as spiral arms.

How a wave of star formation propagates through theinterstellar medium. Successive waves continually recreatethe bright stars and HII regions that trace out the spiral arms.

Both theories havestrong points (andweak points), perhaps both processes play a role in our galaxy.

If we could see ourgalaxy face-on, it would look very much like this.

Note the color difference between disk, nuclear bulge,and core. HIIregions show as redpatches in the arms.

NGC 7331 is thought to be similar to our galaxy.

NGC 7331 in IR showing dust and star formation regions.

If we could see our Galaxyedge-on from the outside, it would look like this.

The Milky Way Galaxy contains roughly 100 billion stars and our Sunis one star among these, located roughly 30 thousand light years from the center of the Galaxy in the disk.

What about other galaxies?