The Milky Way

Announcements Assigned reading: Chapter 15.1 Assigned reading: Chapter 15.1 Please, follow this final part of the course with great care Please, follow this final part of the course with great care It is the most difficult one, less intuitive one It is the most difficult one, less intuitive one Lots of new notions Lots of new notions

The Milky Way, Our Galaxy What is a galaxy What is a galaxy Galaxies as the building blocks of the Universe Galaxies as the building blocks of the Universe The bottom line about galaxies: visible matter and dark matter The bottom line about galaxies: visible matter and dark matter The basic structure of the Milky Way Galaxy The basic structure of the Milky Way Galaxy The-star-gas-star cycle The-star-gas-star cycle The motion of the Milky Way The motion of the Milky Way Super-massive black hole in the center Super-massive black hole in the center

How Galaxies are really made Dark matter outweighs visible matter by 1 to 10 It is the dominant source of gravity in the Universe The Universe is made of Dark Matter Visible Matter is only the tip of the iceberg Yet, even if we detect Its presence, we still do not know what Dark Matter is made of!

The Big Questions What is the shape of the Milky Way galaxy? What is the shape of the Milky Way galaxy? How do we know where we are in the Galaxy? How do we know where we are in the Galaxy? What wavelengths of radiation effectively penetrate the dusty interstellar medium? What wavelengths of radiation effectively penetrate the dusty interstellar medium? How do we know the rotating structure of the Galaxy? How do we know the rotating structure of the Galaxy?

Another Galaxy: the Andromeda Galaxy About 2 million light years away. Angular size (about 2 degrees) ---> the size of the Milky Way. Another island universe!

To study the structure of the Milky Way, we need to measure distances to stars There are well-tested methods for measuring distances over short length scales: Radar ranging - good for measuring distances in the solar system (up to about light years) Radar ranging - good for measuring distances in the solar system (up to about light years) Parallax - good for measuring distances to a few hundred light years Parallax - good for measuring distances to a few hundred light years

But what do we do about objects too far away to use radar ranging or parallax?

Standard Candles If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness. If we know an object’s true luminosity, we can measure its distance by measuring its apparent brightness. An object that has a known luminosity is called a standard candle.

Standard Candle #1 - Cepheid Variable Stars Cepheid variable stars have variable brightness that is very regular. Cepheid variable stars have variable brightness that is very regular. The period of the variation can be from days to weeks – and it seems to be a reliable indication of the star’s luminosity! The period of the variation can be from days to weeks – and it seems to be a reliable indication of the star’s luminosity! Important!! Go refresh Capter 12, Section 4. Important!! Go refresh Capter 12, Section 4.

Cepheid Variable Stars Henrietta Leavitt Henrietta Leavitt ( ). Luminosity=4  D 2 B

1920 Harlow Shapley Observed that the globular star clusters were centered about a point that was displaced from the Sun. Shapley proposed that the point was the center of the Milky Way. Harlow Shapley's diagram of the distances of the globular clusters from the Sun.

Globular Clusters and Understanding our Galaxy The globular clusters in the halo of the Milky Way have told us two important things about our own galaxy: The globular clusters in the halo of the Milky Way have told us two important things about our own galaxy: The Sun is not at the center of the galaxy The Sun is not at the center of the galaxy The galaxy is a much larger system than it appeared based on early observations The galaxy is a much larger system than it appeared based on early observations

The Anatomy of (the light-emitting part of) the Galaxy

Basic components: Thin disk Bulge Spiral arms halo

The Galaxy

Key Parts: - disk: supported by rotation - halo: supported by motion pressure - bulge: supported by both rotation and motion pressure Disk Halo Bulge Globular clusters

Rotation of the disk

Disk supported by rotation Halo supported by random motion Bulge supported by random motion and small rotation

Dust is generated in the late stages of low and high mass stars, when carbon and silicon is dredged up from the cores and ejected in stellar winds, planetary nebulae, and possibly supernova remnants. The blocking of visible light by dust is called dust extinction. Dust – a hindrance to our study of the Milky Way A view at visible wavelengths of the galactic plane.

A Reminder About Scattering If the dust is thick enough, visible light is absorbed (or scattered) and only the longer wavelengths get through.

Radio Microwave Infrared Visible UV X-ray longer wavelength (redder) shorter wavelength (more blue) Blocked by Interstellar Dust

So, to examine our own galaxy, we must use Radio, mm-wavelength, infrared, and X- ray telescopes to peer through the interstellar medium. Very Large Array Chandra X-ray Observatory

Infrared view of the sky

Survey Questions What is the shape of the Milky Way galaxy? What is the shape of the Milky Way galaxy? How do we know where we are in the Galaxy? How do we know where we are in the Galaxy? What wavelengths of radiation effectively penetrate the dusty interstellar medium? What wavelengths of radiation effectively penetrate the dusty interstellar medium?

Survey Questions How do we measure the motion of the Milky Way? How do we measure the motion of the Milky Way? How is the disk of the Milky Way supported? How is the disk of the Milky Way supported? How is the bulge supported? How is the bulge supported? How do we know there is a super-massive black hole in the center of the Milky Way? How do we know there is a super-massive black hole in the center of the Milky Way?

Radio Observations are key to understanding the Disk. Very Large Array Interstellar hydrogen emits strongly at 21cm wavelengths.

A full sky image of hydrogen (21 cm emission) By looking at the Doppler Shift of the 21 cm emission, we can reconstruct the distribution of objects in the galaxy.

Looking for 21-cm wavelengths of light … Looking for 21-cm wavelengths of light … emitted by interstellar hydrogen emitted by interstellar hydrogen as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts as we look along the disk of the Milky Way (from inside), we see 21-cm photons Doppler shifted varying amounts this allows the velocity and mass of interstellar hydrogen to be mapped this allows the velocity and mass of interstellar hydrogen to be mapped Radio observations help map the galactic disk You are here

A Map of the Milky Way Based on 21-cm wavelength light mapping

Spiral Galaxy M83 observed in both visible light and radio wavelengths.

The dominant structures in the disk are the spiral arms. Spiral arms are density waves that move at different velocities from the stars. The Nature of the Spiral Arms What is a density wave?

The gas and stars in the galaxy rotate at a different rate than the spiral arms (density waves)

Survey Question We find mostly hot, massive stars in the spiral arms of galaxies because 1) hot, massive stars are preferentially produced in the spiral arms 2) less massive stars live long enough to rotate out of the spiral arms 3) supernovae destroy the less massive stars in the spiral arms 4) there is too high a density in the spiral arms to create low-mass stars

The Star-Gas-Star Cycle There is a continuous reprocessing of gas in the galaxy into stars. Stars form from dense gas in molecular clouds Stars form from dense gas in molecular clouds Stars age and then give up their outer layers (via solar wind, planetary nebula, or supernova) Stars age and then give up their outer layers (via solar wind, planetary nebula, or supernova) The ejected gas eventually finds its way back into an overly dense region and become part of the next generation of stars. The ejected gas eventually finds its way back into an overly dense region and become part of the next generation of stars. This process is repeated as long as there is enough hydrogen around to create new stars. This process is repeated as long as there is enough hydrogen around to create new stars.

What characteristic of a star would imply that many star generations preceded it? 1) a high hydrogen abundance 2) a high helium abundance 3) a high “metal” abundance 4) a low “metal” abundance 5) a low helium abundance

Chandra survey of the Galactic center Red: 1-3 keV Green: 3-5 keV Blue: 5-8 keV Wang et al. (2002)

X-ray Flare from Sgr A* Baganoff et al. (2003)

Super Massive Black Holes

Our Galactic Center More than 5000 km/s at a mere 17 light hours distance -- about 3x the size of our solar system. MPE: 4 million solar masses within this distance.

Three key things to keep in mind about the Milky Way 1) The Milky Way is an ecosystem for stars. 2) The Milky Way is mostly empty space … but it is rather dusty. 3) The Milky Way barely moves at all on the scale of a human lifetime.

Discussion questions 1) What wavelengths of radiation effectively penetrate the dusty interstellar medium? Radio and Infrared what velocity and what period? –vel. = 220 km/s, period = 240 million years True/False - The same stars move along with the spiral density waves all the time. –False, the stars orbit the galactic center at a different rate than the spiral density waves.

Survey Questions How do we measure the motion of the Milky Way? How is the disk of the Milky Way supported? How is the bulge supported? How do we know there is a super-massive black hole in the center of the Milky Way?