The Milky Way

Announcements Assigned reading: Chapter 15 Assigned reading: Chapter 15 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 Almost everything we see in the night sky belongs to the Milky Way. We see most of the Milky Way as a faint band of light across the sky. From the outside, our Milky Way might look very much like our cosmic neighbor, the Andromeda galaxy.

The Milky Way, Our Galaxy Our Sun is part of a large systems of other stars called a “Galaxy”. Our Sun is part of a large systems of other stars called a “Galaxy”. The name of our galaxy is “Milky Way” (the word “galaxy” in Greek means “milky way” The name of our galaxy is “Milky Way” (the word “galaxy” in Greek means “milky way” What is a galaxy? What is a galaxy? Galaxies are systems made of dark matter, stars, gas, dust Galaxies are systems made of dark matter, stars, gas, dust Galaxies are the building blocks of the Universe Galaxies are the building blocks of the Universe The bottom line about galaxies: made by baryonic, i.e. normal matter (stars, gas and dust), and dark matter (unknown nature) The bottom line about galaxies: made by baryonic, i.e. normal matter (stars, gas and dust), and dark matter (unknown nature) Let’s start by studying a special galaxy, our own: The Milky Way Let’s start by studying a special galaxy, our own: The Milky Way The basic structures of the Milky Way Galaxy: bulge, disk, halo The basic structures of the Milky Way Galaxy: bulge, disk, halo What happens inside the Milky Way: the-star-gas-star cycle What happens inside the Milky Way: the-star-gas-star cycle The motions of the Milky Way: how they happen and where; What do they mean, and what they are useful for? The motions of the Milky Way: how they happen and where; What do they mean, and what they are useful for? The mysterious center of the MW: the super-massive black hole The mysterious center of the MW: the super-massive black hole

How Galaxies are really made Dark matter outweighs visible matter by 10 to 1 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 Nearest Bright Galaxy: the Andromeda Galaxy About 2 million light years away. Angular size (about 2 degrees) ---> the size of the Milky Way. Another island universe!

The Universe is filled of galaxies: they are the building blocks

Often galaxies are found in clusters

The Big Questions What is the shape of the Milky Way galaxy? What is the shape of the Milky Way galaxy? How big is it? What is its shape? How big is it? What is its shape? Does it move? Why does it not collapse? Does it move? Why does it not collapse? 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?

How do we explore the Galaxy and establish its shape and size? Since a galaxy is a system of stars, one way is to just count fainter and fainter stars in all directions. Since a galaxy is a system of stars, one way is to just count fainter and fainter stars in all directions. Once I reach the most distant stars (which appear as the faintest) at the edge of the system, I find no more fainter stars. Once I reach the most distant stars (which appear as the faintest) at the edge of the system, I find no more fainter stars. By repeating this procedure along all directions, I can get an idea of the shape of the stellar system. By repeating this procedure along all directions, I can get an idea of the shape of the stellar system. Once I measure the distance to the stars, I also estimate the size. Once I measure the distance to the stars, I also estimate the size. Herschel and Kapteyn did just that and other things, and concluded that the Galaxy was a flattened disk (correct), with the Sun at the center (wrong), and with a radius of about 5 kpc (wrong, too little). Herschel and Kapteyn did just that and other things, and concluded that the Galaxy was a flattened disk (correct), with the Sun at the center (wrong), and with a radius of about 5 kpc (wrong, too little). They got the wrong answers because they were looking through dust and could only see the nearest stars. They got the wrong answers because they were looking through dust and could only see the nearest stars. It is as when we look around us in the middle of a think fog: all we can see are the nearest things around us. It is as when we look around us in the middle of a think fog: all we can see are the nearest things around us.

The Structure of the Milky Way Galactic Plane Galactic Center The structure is hard to determine because: 1) We are inside 2) Distance measurements are difficult 3) Our view towards the center is obscured by gas and dust.

First Studies of the Galaxy First attempt to unveil the structure of our Galaxy by William Herschel (1785), based on optical observations The shape of the Milky Way was believed to resemble a grindstone, with the sun close to the center.

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 and 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 In fact, too much dust can block light altogether, especially UV, Optical and near-Infra-Red light. This happens in the disk of galaxies

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. Standard Candels #2: Type Ia supernovae

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

Exploring the Galaxy Using Very Bright Objects: Clusters of Stars Two types of star clusters: 1) Open clusters: young clusters of recently formed stars; within the disk of the Galaxy Open clusters h and  Persei 2) Globular clusters: old, centrally concentrated clusters of stars; mostly in a halo around the Galaxy Globular Cluster M 19

Globular Clusters Dense clusters of 50,000 – 1 million stars Old (~ 11 billion years), lower-main- sequence stars There are approx. 200 globular clusters in our Milky Way Globular Cluster M80

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. I. Thin disk: young stars (Pop I), gas, dust, metal rich associations, young clusters Bulge: old and young stars (Pop I&II), glob. clusters, some gas, SMBH Spiral arms: young stars (Pop I), gas, associations, open clusters Halo: old stars (Pop II), metal poor, glob. clusters

Basic Components. II.

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

Disk supported by rotation Halo supported by random motion Bulge supported by random motion and small rotation Suggests formation scenario: Bulge old: rapid fragmentation and collapse of gas Disk young: progressive collapse of gas, dissipation and formation of rotating disk, star formation

How massive is the Galaxy? To measure mass, we need orbital motions To measure mass, we need orbital motions Sun orbits Center of Galaxy at v~220 km/s and distance r~8.5 kpc Sun orbits Center of Galaxy at v~220 km/s and distance r~8.5 kpc Circonference is 2  r; at that speed period of rotation is 240 million years Circonference is 2  r; at that speed period of rotation is 240 million years Thus, mass inside the Sun orbit is: Thus, mass inside the Sun orbit is: M Sun = a 3 / p 2 ~ 0.9 x M o M Sun = a 3 / p 2 ~ 0.9 x M o Including the mass outside the Sun, one actually finds: Including the mass outside the Sun, one actually finds: M Tot ~ 4 x M o M Tot ~ 4 x M o However, mass in star (from star counting) is much less: However, mass in star (from star counting) is much less: M Star ~ 0.8 x M o M Star ~ 0.8 x M o

Rotation of the disk This is verified only if all the mass is at the center, like in the Solar system. If there is mass at all radii, then the rotation curve does not decrease; it might stay flat or even increase! We measure the mass of galaxies using rotation curves

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.

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

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 using X-ray light 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)

Our Galactic Center More than 5000 km/s at a mere 17 light hours distance -- about 3x the size of our solar system. MPE: The dark object has an implied mass of 4 million times the mass of the Sun within this distance.

Super Massive Black Holes (SMBH) We now believe that all galaxies harbor a SMBH at their center. When the SMBH actively accretes gas it liberates an enormous. amount of energy (AGN, Quasars). Life would probably not be possible if our SMBH wakes up and becomes a Quasar. Astronomers discovered that the larger the mass in stars around a SMBH, the larger the SMBH itself. There must be an intimate connection between the formation of stars in a galaxy and the formation of the SMBH, but nobody knows what this is. Maybe SMBH are something intrinsic, or even essential, to the process of galaxy formation.

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 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?