The Milky Way Galaxy

Milky Way Galaxy

Edge-On View of the Milky Way

Periodic Variable Stars - The Next Step in Distance Measurement

• When certain larger stars move from the main sequence stage to the red giant stage, they pass through the “instability strip” on the HR diagram

• While in the strip, these stars pulsate in size, temperature, and luminosity in a regular way

RR Lyrae and Cepheid Variables

• RR Lyrae Variables are generally found in older star clusters such as globular clusters. Their period of pulsation is generally less than one day. They all have about the same absolute magnitude, M=+0.5.

• Cepheid Variables are generally found in newer star clusters such as galactic clusters. Cepheids are observed to have periods ranging from 1 to 100 days. They are brighter than Lyraes with absoute magnitudes of M=-2.5 and brighter. They follow the period-luminosity law – the brighter Cepheids have longer periods.

Variable Star Light Curves

• RR Lyrae and Cepheid variable stars are recognizable by the characteristic shapes of their light curves

• Pulsations occur when the stars experience a brief period of instability due to obstruction to the normal flow of radiation to the surface

Variable Stars as Distance Indicators

• If M can be predicted and if m is known, the distance d can be calculated by: M=m-5log(d/10)

• Lyrae variables all have M=+0.5

• Cepheids follow the period-luminosity law. If the period is measured, the graph predicts the absolute brightness

Variable Stars on the Distance Ladder

• Lyrae variables are useful for measuring distances around our Milky Way galaxy

• Cepheid variables are useful for measuring distances to nearby galaxies

Globular Clusters

• First determination of sun’s position in the Milky Way was made by Harlow Shapley (about 1915). He recorded distances and directions to a number of globular clusters.

• RR Lyrae Variables were used as distance indicators

• He deduced that center of spherical distribution of these clusters is center of Milky Way

Stellar Populations

• Galactic disk stars are labeled Population I stars (metal rich). The disk is an active star formation region, so the spectrum of Population I stars shows more abundance in heavy elements than halo stars. Remember, the spectrum comes from the atmosphere of the star.

• The halo stars are labeled Population II stars. Their spectrum shows they are metal poor. They were formed long ago when the heavy metal concentration in the galaxy was low.

• The galactic disk appears bluish (newer stars) in color while the halo is reddish (older stars)

Orbital Motion

• All matter in the galaxy must be in motion

• The sun (8 kpc from the center) takes about 225 million years to complete an orbit

• The rotation period is shorter for material closer than the sun and longer for material further out

• Orbital orientations of halo stars are random

Formation of the Milky Way

• Halo stars were the first to form in the galaxy about 15 billion years ago

• The galaxy flattened into its present shape due to initial rotation of the cloud

• New stars form today where there is interstellar matter - in the disk

Tracing the Spiral Arms

• Optical tracers of the spiral arm structure of the Milky Way include O and B stars, galactic clusters, HII regions, and Cepheid Variables. Dust and gas limits the optical view to a distance less than about 5000 pc.

• The most extensive mapping of the spiral arm structure has come from observing HI regions using 21 cm radiation. Long wavelength waves are largely unaffected by interstellar dust.

• Radio signals from molecules identify the locations of the denser interstellar clouds

Density Wave Theory

• If the stars closer to the center go around in a shorter period of time, why don’t the spiral arms wind up on themselves?

• Density wave theory predicts that stars orbit faster outside the spiral arms but slow down when they get to the spiral arm region

• Cause: lumpiness in the mass distribution of the galaxy

• Another theory: shock waves from star formation create the spiral arm structure

Spiral Arms

• Star formation occurs at the leading edge of the spiral arms

• As the stars leave they speed up and move ahead of the arm

• The arms also rotate as a fixed feature of the galaxy

Mass of the Milky Way

• What “object” in the Milky Way does the sun orbit?

• The sun effectively orbits the combined mass of all stars and interstellar material that lie inside the sun’s orbit. The material outside the sun’s orbit has no effect.

• Using the orbital distance and orbital period of the sun in Kepler’s 3^rd Law, the mass of the galaxy inside the sun’s orbit is calculated to be 100 billion solar masses

• The mass of a galaxy at any distance can be determined by studying its rotation curve

• From studies of orbiting matter at 40 kpc astronomers calculate the mass of the Milky Way galaxy inside that distance to be 600x10⁹ solar masses

Rotation Curve of the Milky Way

• The rotation curve shows that the rotation speed does not drop at the “radio edge” of the galaxy (at ~40 kpc). This means that this edge is not the actual edge of the galaxy. It keeps going.

• The luminous matter of the galaxy (out to ~15 kpc) is surrounded by an extensive, invisible dark halo

• Most of the mass of the galaxy exists in the form of dark matter - dwarf stars (Machos) or exotic subatomic particles (WIMPS)?

The Galactic Center

• Astronomers working at infrared and radio wavelengths have uncovered evidence of energetic activity (high velocities and a lot of energy emitted) within a few parsecs of the galactic center

• Radio source Sagittarius A (emitting region ~10 AU in size) is located at the galactic center

• The leading explanation is that a black hole of 2-3 million solar masses resides at the heart of our Milky Way galaxy

Elliptical Galaxies (E)

• No disk or spiral arms

• Stars smoothly distributed through an ellipsoidal volume ranging from nearly spherical (E0) to very flattened (E7)

• Contain only old stars

• Little or no cool gas and dust

• Stars have random orbits in three dimensions

• Dwarf ellipticals (a few million stars) are the most common type. Giant ellipticals can contain trillions of stars

Spiral/Barred Spiral Galaxies

• Highly flattened disk of stars and gas containing spiral arms and thickening to a central bulge. Sa and SBa have the largest bulges. Sc and SBc have the most diffuse spiral arms

• Disk contains new and old stars and substantial amounts of gas and dust

• Halos contain old stars and little gas or dust

• Disk stars move in circular orbits. Halo stars have random orbits in three dimensions

• Andromeda and the Milky Way are Sb galaxies

Irregular Galaxies

• No obvious structure. May even have an explosive appearance.

• Contain both young and old stars

• Abundant in gas and dust

• Vigorous ongoing star formation

• Stars and gas have very irregular orbits

Evolution of Galaxies?

• Isolated normal galaxies do not evolve from one type to another

• Within a given region of space equally old stars are found in all galaxies

• There is now strong observational evidence indicating that collisions and tidal interactions between galaxies may be the main physical process driving galaxy evolution