PART 3 Galaxies. Stars in the Milky Way

Transcription

1 PART 3 Galaxies Stars in the Milky Way A galaxy is a large collection of billions of stars The galaxy in which the Sun is located is called the Milky Way From our vantage point inside the galaxy, the Milky Way looks like a band of stars across the night sky, with dark dust lanes obscuring the center of the band. Our Galaxy, the Milky Way 1

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3 The Milky Way is a spiral galaxy approximately 30 kps (100K Ly) across. The Sun is located around 8 kpc from the center, in one of the spiral arms. Most of the stars are concentrated in the galactic plane, or in the central bulge at the center of the galaxy Inside the bulge is the nucleus of the galaxy Surrounding the disk is a roughly spherical distribution of stars called the halo. Globular clusters are distributed throughout this halo, surrounding the center of the galaxy. 3

4 Two Stellar Populations: Pop I and Pop II Stars in neighboring galaxies are segregated Younger, blue, Population I stars were found mostly in the disks and spiral arms. Typically less than a few billion years old; Follow circular orbits in the galactic plane Older, red, Population II stars were found mostly in the halo and central bulge; More than 10 billion years old; Follow random elliptical orbits around the galactic center not in the plane. The primary reason that massive O-type stars are not found in the galactic halo is because they are a) too massive to be kicked into the halo from the disk. b) so massive that they settle into the thinner disk. c) too short-lived to have persisted from halo formation until today. d) closer to us in the disk than in the extended halo. 4

5 The Formation of the Milky Way Our galaxy likely began 13 billion years ago as a huge cloud of pure hydrogen and helium, slowly rotating and collapsing The first stars formed (Pop III?) within this cloud, burning out quickly and violently. This added heavy elements to the cloud Population II stars formed next, capturing some of the heavy elements and settling into elliptical orbits around the center of the cloud As the collapse continued, a disk formed, and Population I stars formed from the ashes of dying Pop I stars Compared to stars like the Sun in the disk of the Milky Way, stars that populate the extended spheroidal halo of the galaxy were born a) earlier, so have had time to accumulate more heavy elements. b) later, so have used up their heavy elements. c) earlier, from more nearly primordial material, so have fewer heavy elements. d) later, so have accumulated more heavy elements from previous generations of stars. 5

6 Mapping the Milky Way s spiral arms Once this difference between Population I and II stars was noted, astronomers could map our galaxy s arms Population I stars are mostly bright, blue stars (hot O and B stars) found in the disk By measuring the location of O and B stars near the Sun, the first pictures of the Milky Way s spiral structure were produced. Dust and gas obscure the light from more distant stars, so the map is incomplete. Galactic Cannibalism There are a few observations that are not explained by this model Some stars follow unusual orbits in the galaxy Not all Pop II stars are the same age Model predicts that the first stars might not have been very massive, and should still be around! Galactic cannibalism provides some answers The Milky Way may be absorbing another galaxy! Observations show streams of stars coming from our galaxy s victim. 6

7 The Interstellar Medium The Sombrero Galaxy Space is far from empty! Clouds of cold gas Clouds of dust In a galaxy, gravity pulls the dust into a disk along and within the galactic plane This dust can obscure visible light from stars and appear to be vast tracts of empty space Fortunately, it doesn t hide all wavelengths of light! 7

8 Emission Nebulae We frequently see nebulae (clouds of interstellar gas and dust) glowing faintly with a red or pink color Ultraviolet radiation from nearby hot stars heats the nebula, causing it to emit photons This is an emission nebula! 8

9 Reflection Nebulae When the cloud of gas and dust is simply illuminated by nearby stars, the light reflects, creating a reflection nebula Typically glows blue Dark Nebulae Nebulae that are not illuminated or heated by nearby stars are opaque they block most of the visible light passing through it. This is a dark nebula 9

10 As starlight passes through a dust cloud, the dust particles scatter blue photons, allowing red photons to pass through easily The star appears red (reddening) it looks older and dimmer (extinction) than it really is. Interstellar Reddening If one region of the sky shows nearby stars but no distant stars or galaxies, our view is probably blocked by a) nothing, but directed toward a particularly empty region of space. b) an emission nebula of ionized gas. c) an interstellar gas and dust cloud. d) a concentration of dark matter. 10

11 Rotation in the Milky Way The Milky Way does not rotate like a solid disk! Inner parts rotate about the center faster than outer parts Similar to the way planets rotate around the Sun This is called differential rotation. A plot of rotation speed vs. distance is called a rotation curve A star is held in orbit around the galactic center by gravitational forces of all matter inside its orbit! Rotation in the Milky Way The Milky Way does not rotate like a solid disk! Inner parts rotate about the center faster than outer parts Similar to the way planets rotate around the Sun This is called differential rotation. A plot of rotation speed vs. distance is called a rotation curve A star is held in orbit around the galactic center by gravitational forces of all matter inside its orbit! 11

12 Rotation in the Milky Way The Milky Way does not rotate like a solid disk! Inner parts rotate about the center faster than outer parts Similar to the way planets rotate around the Sun This is called differential rotation. A plot of rotation speed vs. distance is called a rotation curve A star is held in orbit around the galactic center by gravitational forces of all matter inside its orbit! Calculating the Mass of the Galaxy The rotational velocity of the Sun around the center of the galaxy can be used to estimate the galaxy s mass The combined gravitational effect of all mass within the Sun s orbit is equivalent to one large lump of mass at the center of the galaxy Newton s Law of Gravitation shows that the mass of all matter within the Sun s orbit is solar masses! We can estimate the mass of the entire galaxy be measuring the orbital velocity of small satellite galaxies in orbit around the Milky Way ( solar masses) 12

13 If there was no dark Matter in the Milky Way the velocity of the sun around the Galactic Center would be a) greater b) smaller c) the same d) impossible to say The Galactic Center and Edge Despite the appearance of being closely spaced, stars in the Milky Way are very far apart At the Sun s distance from the center, stellar density is around 1 star per 10 cubic parsecs Density is much higher at the core Exceeds 100,000 stars per cubic parsec! X-ray and gamma ray telescopes reveal a supermassive black hole at the Milky Way s core Called Sag A* 5 million solar masses! 13