Galaxies

1. Galaxies

2. Our Galaxy • We live in the Milky Way galaxy • It is made up of about 100 billion stars arranged in a spiral shape • The sun is located on one of these arms, far from the nucleus of our galaxy. • If the sun was in a different part of the galaxy, would that affect the formation of our planet and complex life? (Discuss)

3. From the Side • If we could see it from the side, it would look like this – a disk with a central nuclear bulge • Around it a halo of stars occur which are not in the central disk or nuclear bulge • There is probably a black hole in the center and a lot of dark matter in the halo • Could a planet like earth exist near the galaxy’s center? discuss Dark matter is material which does not give off radiation, so it is invisible. But it has gravity and thus effects other matter. We will discuss black holes in another chapter.

4. From here • From earth, all we can see of our galaxy is a picture like this, since we are inside of it. (the photo at the left.) We are looking up at the disk towards the center of our galaxy. • The picture at the right was composed by using actual satellite data to synthesize a side view of our galaxy • How do we know the Milky Way is spiral? Link

5. Population I and II stars Pop II • The stars in the Milky Way are made of two types – these are called Population I and II stars • The Population I stars are in the disk and arms and contain some heavy elements. Our sun is a Population I star. • Population II stars are in the nuclear bulge and halo and contain only traces of heavy elements. • Why the difference? What do you think? Pop I The Population II stars are older. They formed earlier when there were less heavy elements. The arms and disk are regions where stars formed later (some are still forming) when heavier elements from earlier star formations and supernovas had “seeded” the universe with heavier elements. Do you think complex life could form on a planet around a Population II star?

6. Formation of Milky Way • So the evidence seems to say that the nuclear bulge formed first, and the arms later. Arms could form from gravitational tugs from other galaxies, or from supernova disturbances • But astronomers are unsure how the arms are maintained. Think about it – If you were spinning with your arms out, what would happen to your arms? • So, how are arms maintained? • There are at least two hypotheses: • 1) A density wave, sort of like a truck in traffic. Imagine you are behind the truck. What happens to the cars? Perhaps stars can bunch up too. • 2) The arms continually break up and reform

7. How many others • Of course the Milky Way is not the only galaxy. How many others are there? • The Hubble telescope took a picture of a small section of the sky above us. (Explain) • Hubble Deep Field video • Apparently there are billions of galaxies, each with billions of stars. This bodes well for the possibility of other life – Many, many stars!! • How do we find out about them? Stop to discuss/demo stellar magnitudes (dittos) – absolute and apparent.

8. Cepheid Variables • Variable stars were discovered in our galaxy and were originally used by astronomers to calculate the distances to the globular clusters in our galaxy • Variable stars are giant stars with varying magnitudes. • In some variable stars, like Cepheid variables, there is a direct relationship between the period of this variation and the luminosity (brightness) of the star. So, astronomers can calculate the luminosity, which gives them the absolute magnitude of the star • Then they used the absolute magnitude along with the apparent magnitude to find the distance to the star • Cepheids and distance video

9. Variable graphs • The top graph shows the light intensity variation of a variable star known as R-Scooti (not a Cepheid) • What do you think is the likelihood of life developing on a planet around R-Scooti? • The bottom graph shows the period vs. absolute magnitude relationship for Cepheid variable stars. • Let’s try a problem using the 2nd graph.

10. How did this relate to galaxies? • Astronomers were unsure what some of the other shapes in the sky were – Were they gas collections in our galaxy? Or something else? • In 1924 the famous American astronomer Edwin Hubble found Cepheid variables in the Great Nebula in the Andromeda constellation. Link • He used these variables to calculate a distance, as previously discussed. • He discovered that the distance was over a million light years – too far to be in our galaxy LINK • So, the nebula was renamed the Andromeda Galaxy – one of our closest neighbors. You can see this fuzzy patch in the constellation Andromeda, if you look carefully.

11. Classification of Galaxies • Hubble also classified the galaxies by their shapes • He was trying to come up with a system for figuring out how different galaxies formed, and their relative ages • Although the formation idea didn’t pan out, the classification scheme is still used • Let us try our own classification scheme activity (CP) before we go on.

12. Hubble’s Classification • Hubble’s classification scheme is shown below • How did yours compare? • How would you classify the Milky Way? • Could we exist in an elliptical galaxy?

13. Now classify these! spiral elliptical barredspiral irregular

14. Hubble classification examples • The link below shows some real examples of galaxies which illustrate different groups of Hubble’s classification scheme • Classzone Hubble classification hyperlink • Video - review of galaxies

15. Our place in the universe • There are many other galaxies grouped into clusters and superclusters • The link below shows us where our galaxy is in relationship to the rest of the universe • Classzone hyperlink

16. Size and Scale of the Universe Image courtesy of The Cosmic Perspective by Bennett, Donahue, Schneider, & Voit; Addison Wesley, 2002

17. The Solar System • 8.5 planets, thousands and thousands of planetoids and asteroids, billions of comets and meteoroids • Mostly distributed in a disk about the Sun • Sun blows a constant wind of charged gas into interplanetary space, called the Solar Wind Boundary between Solar Wind and interstellar space at 100 AU from the Sun (200 AU diameter)

18. The Solar Neighborhood • The region of the Galaxy within about 32.6 light-years of the Sun (65 light-years diameter) is considered its neighborhood. • Here stars move generally with the Sun in its orbit around the center of the Galaxy • This region is inside a large bubble of hot interstellar gas called the Local Bubble. Here the gas temperature is about 1 million degrees Kelvin and the density is 1000 times less than average interstellar space. To Center of Galaxy The image is 390 light-years across. Direction of Galactic Rotation

19. You Are Here The Milky Way Galaxy The Milky Way Galaxy is a giant disk of stars 160,000 light-years across and 1,000 light-years thick. The Sun is located at the edge of a spiral arm, 30,000 light-years from the center It takes 250 Million years for the Sun to complete one orbit There are over 100 Billion stars in the Milky Way The Spiral arms are only 5% more dense than average, and are the locations of new star formation

20. The Local Group • Contains 3 large spiral galaxies--Milky Way, Andromeda (M31), and Triangulum (M33)—plus a few dozen dwarf galaxies with elliptical or irregular shapes. • Gravitationally bound together—orbiting about a common center of mass • Ellipsoidal in shape • About 6.5 million light-years in diameter

21. A cluster of many groups and clusters of galaxies • Largest cluster is the Virgo cluster containing over a thousand galaxies. • Clusters and groups of galaxies are gravitationally bound together, however the clusters and groups spread away from each other as the Universe expands. • The Local Supercluster gets bigger with time • It has a flattened shape • The Local Group is on the edge of the majority of galaxies • The Local Supercluster is about 130 Million light-years across The Local Supercluster

22. 1.3 Billion light-years The Universe • Surveys of galaxies reveal a web-like or honeycomb structure to the Universe • Great walls and filaments of matter surrounding voids containing no galaxies • Probably 100 Billion galaxies in the Universe The plane of the Milky Way Galaxy obscures our view of what lies beyond. This creates the wedge-shaped gaps in all-sky galaxy surveys such as those shown here.

23. Galaxy clusters As shown on the previous slides • Galaxies are grouped in clusters. Groups of Galaxies are called clusters • The Milky Way is in The Local Group, a small cluster of about 35 galaxies. Most are small. The Milky Way and Andromeda galaxies are the largest. • These clusters are grouped into superclusters, appearing in sheet-like shapes This is the Hercules cluster

24. Collisions • Within clusters collisions can occur • Here are some examples - • The Milky Way will collide with Andromeda. They are being pulled together by gravity • But don’t worry – it is billions of years in the future. Do you think that our earth would be different if we were located in a more crowded cluster?

25. Quasars & AGNs – odd objects • Quasars exist in the center of some galaxies • They may be caused by black holes in the center of galaxies • They probably are temporary and are early formation events • Link • The Milky Way is an example of a typical, “normal” galaxy. • But some galaxies look very different. They emit enormous amounts of energy, much of it is in the form of radio waves and X-rays, which are not coming from the stars. For this reason they are called “active galaxies.” • It is still not yet entirely clear what is producing this energy, but the most widely accepted model is that it is a massive Black Hole (about 100 million times the mass of the Sun) that sits at the center of the galaxy. The Black Hole and its surrounding material are known as an Active Galactic Nucleus, or an AGN for short.

26. My favorite galaxies–the Sombrero galaxy – in visible light and infrared And this one! What type is it? LINK