This Week in Astronomy

Transcription

1 Homework #8 Due Wednesday, April 18, 11:59PM Covers Chapters 15 and 16 Estimated time to complete: 40 minutes Read chapters, review notes before starting

2 This Week in Astronomy Credit: NASA/JPL-Caltech A Very Fast Supernova! M. Pursiainen / University of Southampton and DES collaboration Animation of KSN 2015K exploding as a supernova event. Very fast rise time of light curve thought to be due to a cocoon of material that surrounded the star prior to explosion.

3 What lies in the center of our galaxy?

4 Stars appear to be orbiting something massive but invisible a black hole? Orbits of stars indicate a mass of about 4 million M sun.

5 Milky Way s central black hole has a mass of M BH = 4 million M Sun (a bit low-weight as central black holes go; most massive are 10 billion M Sun ) These supermassive black holes are believed to occur only at the centers of galaxies. Black hole is only a tiny fraction of the mass of the entire Galaxy (weight of your hair vs. weight of your body) and only affects stars very near it.

6 X-ray flares from the galactic center suggest that tidal forces of the suspected black hole occasionally tear apart chunks of matter about to fall in.

7 What would happen to the Earth/Sun if the Milky Way s central black hole suddenly disappeared? A) The Earth/Sun system would move toward the center of the Galaxy. B) The Earth/Sun system would fly away, since the mass of the black hole was gone. C) The Earth/Sun system would be basically unaffected. D) The Earth/Sun system would move into the halo of the Galaxy.

8 What would happen to the Earth/Sun if the Milky Way s central black hole suddenly disappeared? A) The Earth/Sun system would move toward the center of the Galaxy. B) The Earth/Sun system would fly away, since the mass of the black hole was gone. C) The Earth/Sun system would be basically unaffected. D) The Earth/Sun system would move into the halo of the Galaxy. The mass of the black hole is tiny compared to the Galaxy s mass only stars very near the black hole would be affected.

9 Chapter 15 Study Guide 1) The Milky Way is both a band of light across the sky and the name of the galaxy in which we reside. 2) Much of the light from our galaxy is obscured by dust/gas led early astronomers to assume we were in the center of it (wrong). 3) Our galaxy is composed of (know geometry) : Sun disk: thin/flat, old+new heavy element-rich stars, gas/dust bulge: central/spherical, old stars, no gas/dust halo: spherical/large, old heavy element-poor stars, no gas/ dust globular clusters: compact star cluster, very old, orbit our galaxy in halo, about 150 of them.

10 Chapter 15 Study Guide 4) Sun is on 230 million year merry-go-round orbit around center of galaxy at a distance of 27,000 light years. 5) Can use velocity, distance of Sun to measure mass of Galaxy using Newton s gravity law at least 100 billion M Sun. 6) Star gas star cycle recycles gas: stars blow winds and/or blow up hot gas cools neutral gas molecular gas star formation repeat (see Summary of Galactic Recycling slide) 7) Star formation sites can be identified by presence of ionization nebulae (gas lit up by UV light from young, hot stars) found in spiral arms of the disk, but not in halo or bulge. 8) Disk is where all the action is: gas/dust/star formation.

11 Chapter 15 Study Guide 10) Spiral galaxies probably formed like individual stars form, but on a much larger scale huge cloud collapses, forms a disk 11) Orbits of stars in the center of Milky Way bulge indicates the presence of a 4 million M Sun black hole in the center. Mass of black hole is very small compared to mass of entire Milky Way galaxy. 12) All galaxies probably harbor supermassive black holes in their centers.

12 Chapter 16 A Universe of Galaxies

13 How large is the Observable Universe? Sun Pluto Neptune Uranus Earth Venus Mercury 590m 450m 287m 15m 11m 6m If the Milky Way is the size of a football stadium how big is the observable Universe? 1:10 19 scale Larger than the Pacific Ocean!

14 Galaxies and Cosmology A galaxy s age, its distance, and the age of the universe are all closely related. The study of galaxies is thus intimately connected with cosmology the study of the structure and evolution of the universe.

15 What are the three major types of galaxies?

16 Hubble Ultra Deep Field 1 million second exposure Nearly every point of light is a galaxy with billions of stars each.

17 Hubble Ultra Deep Field

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23 Disk Component: stars of all ages, merry-go-round orbits many gas clouds, active star formation Spiral Galaxy Spheroidal Component: Bulge/halo/globular clusters, old stars, few gas clouds, 3-D orbits In general we call the bulge +halo+globular clusters of a spiral galaxy the spheroidal component.

24 Disk Component: stars of all ages, many gas clouds Blue-white color indicates ongoing star formation Spheroidal Component: bulge/halo/ globular clusters, old stars, few gas clouds Red-yellow color indicates older star population

25 Elliptical Galaxy: All spheroidal component, with no disk component No blue stars, no cold gas, no dust, no recent star formation Red-yellow color indicates older star population.

26 Elliptical Galaxy: All spheroidal component, virtually no disk component Random, 3-D (beehive) stellar orbits, like a giant spiral bulge No recent star formation red and dead galaxies

27 For decades it was believed that ellipticals did not contain a gaseous interstellar medium (ISM). Very little if any cold molecular or cool neutral gas. Where did all the gas from stellar mass loss (planetary nebula + white dwarf supernova) go?

28 Elliptical Galaxies X-ray binaries Hot gas Optical X-ray Elliptical galaxies lack a cold interstellar medium (no star formation), but can have large amounts of HOT (10 million K), X-ray emitting ISM. The gas is too hot to form stars.

29 Irregular Galaxy: Neither spiral nor elliptical. Blue-white color indicates ongoing star formation.

30 You observe a galaxy with significant recent star formation. What type of galaxy can it not be? A) spiral galaxy B) irregular galaxy C) elliptical galaxy

31 You observe a galaxy with significant recent star formation. What type of galaxy can it not be? A) spiral galaxy B) irregular galaxy C) elliptical galaxy Remember, elliptical galaxies do not have much cold gas/dust little of any ongoing star formation (this is why they do not appear blue in photographs). They are red and dead.

32 How are galaxies grouped together?

33 Spiral galaxies are often found in groups of galaxies (up to a several dozen galaxies per group). They are gravitationally bound together. Our Milky Way belongs to the Local Group.

34 The Local Group of Galaxies

35 Elliptical galaxies are much more common in huge clusters of galaxies (hundreds to thousands of galaxies). Most galaxies in clusters are ellipticals.

36 How do we measure the distances to galaxies?

37 Brightness alone does not provide enough information to measure distance (for both stars and galaxies). Can t distinguish near/low luminosity from distant/high luminosity based on brightness alone. This makes distance determinations crucial.

38 We measure galaxy distances using a chain of interdependent techniques: the cosmic distance ladder Cepheids Main sequence fitting Parallax Radar

39 Step 1 (Radar) Determine size of Solar System using radar. Determining the distance to the Sun (1 astronomical unit or AU = 150 million km) is particularly important (as we will see in Step 2).

40 Step 2 (Parallax) Determine distances of nearest stars out to ~1600 lightyears using parallax Parallax technique Crucial to know the value of 1 AU (from Step 1) in order to utilize parallax method

41 Clusters of stars are useful distance indicators if we know the spectral type of their constituent stars.

42 Hyades Cluster Hyades cluster: close enough that the distance to the stars in the cluster (and therefore the cluster itself) can be found by stellar parallax (from Step 2).

43 Step 3 (Main sequence fitting technique) Apparent brightness of star cluster s main sequence tells us its distance H-R diagram Main sequence fitting technique Example: the G (Sun-type) stars in Hyades are brighter than the G stars in Pleiades The Hyades must be closer

44 Cepheid stars are a special type of pulsating variable, very luminous star (not a main sequence star). The period of the pulsation is directly proportional to how luminous it is longer period = more luminous

45 Cepheid Variable Stars The light curve of this Cepheid variable star shows that its brightness alternately rises and falls over a 50-day period (for example).

46 Cepheid variable stars with longer periods have greater luminosities. Note that we need to know the distances of nearby Cepheid stars (in star clusters) to pin down this relation.

47 Step 4 (Cepheid variable stars) Because the period of a Cepheid variable star tells us its luminosity, we can use these stars as standard candles. Need to calibrate method by finding Cepheid variables in clusters of known distance (Step 3)

48 Step 4 (Cepheid variable stars) Cepheids are luminous enough that we can see them in other nearby galaxies! Allows us to determine distances to nearby galaxies first step in distance ladder that allows us to step outside our galaxy.

49 White dwarf supernovae can also be used as standard candles. Recall: White dwarf supernovae occur when a white dwarf steals too much matter from a companion star and explodes. All white dwarf supernovae reach the same peak luminosity.

50 Peak luminosity at 10 billion Suns!! Can be brighter than entire host galaxy! White dwarf supernovae always peak at the same luminosity can be used as a standard candle

51 Step 5 (White Dwarf Supernovae) Apparent brightness of white-dwarf supernova tells us the distance to its host galaxy. Can use to determine distances up to 10 billion light-years White dwarf supernovae examples Need to calibrate using galaxies that also contain Cepheids (Step 4)

52 Which of the following is the crucial rung of the cosmological distance ladder that allows us to bridge the gap between objects in the Milky Way and objects in other galaxies? A) White dwarf supernovae method B) Cepheid variable stars method C) Parallax distance measurements D) Using radar to find the Earth-Sun distance

53 Which of the following is the crucial rung of the cosmological distance ladder that allows us to bridge the gap between objects in the Milky Way and objects in other galaxies? A) White dwarf supernovae method B) Cepheid variable stars method C) Parallax distance measurements D) Using radar to find the Earth-Sun distance Cepheids are bright enough to be seen in nearby galaxies.

54 What is Hubble s law?

55 The Puzzle of Spiral Nebulae Before Edwin Hubble (circa 1920), some scientists argued that spiral nebulae like the Andromeda Galaxy were entire galaxies like our Milky Way, whereas other scientists maintained they were smaller collections of stars within the Milky Way. The debate remained unsettled until someone finally measured the distances of spiral nebulae.

56 Edwin Hubble settled the debate by measuring the distance to the Andromeda Galaxy using Cepheid variables as standard candles.

57 Distance to the Andromeda Galaxy turned out to be over 2 million light years spiral nebulae were galaxies!

58 Hubble also knew that the spectral features of virtually all galaxies are redshifted they re all moving away from us.

59 Discovering Hubble's Law By measuring distances to galaxies (Cepheid variable method Step 4), Hubble found that redshift (velocity) and distance are related in a special way.

60 The farther away a galaxy (larger distance) is, the faster it is moving away from us (larger velocity). Hubble s law: velocity = H 0 distance Hubble s constant (a constant of nature)

61 Redshift of a galaxy tells us its distance through Hubble s law: distance = velocity H 0 Redshift is minor for nearby galaxies, but significant for distant galaxies. If you can measure the velocity of a a galaxy, you can learn its distance!