Galaxies

1. Galaxies Stellar Neighborhoods

2. What are Galaxies? • Galaxies are • Huge • Held together by gravity • Comprised of… • Stars • Objects that orbit those stars • The left over gas and dust from the formation of those objects.

3. What are Galaxies? (cont) • Galaxies rotate. • Stars orbit the center of galaxies at speeds ranging from 210-240km/s (470,000-540,000mph) • Galaxies are where the matter is. • Areas in between galaxies have a density of less than one atom per cubic meter. • The air we breath contains 5,000,000,000,000,000,000,000,000 atoms per cubic meter. (5x1024 or 5 septillion)

4. How many galaxies? • Scientists can estimate that there are about 100,000,000,000 galaxies (1x1011 or 100 billion)

5. How Did Galaxies Form • Galaxies formed as rotating clouds of gas, compressed under gravity, began forming stars. • The oldest galaxies scientist can see formed 750 million years after the big bang.

6. How Big Are Galaxies? • Galaxies have anywhere between 10 million and 1 trillion stars. • Range in size from 27,000,000,000,000km to 3,000,000,000,000,000km • Expressing distances in space is difficult when we used dimensions created for measuring Earth.

7. Light Year • All light travels the same speed in a vacuum and is the fastest known attainable speed. • 3x108 m/s • A Light Year is the distanced traveled by light over the period of one year. • 9,460,000,000,000 km • Most galaxies are between 3,000 and 330,000 light years across.

8. Structure of Galaxies • Most galaxies have two main parts • Central bulge • Surrounding Halo • Similar in shape to the pogo-ball • Galactic Bulge • Most dense region of galaxy • Located in center of galaxy • Contains super massive black hole • Halo • Thinner, outer region of galaxy

9. Types of Galaxies • Galaxies are grouped according to their shapes. • Elliptical • Spiral • Peculiar

10. Elliptical • An ellipse is like a specific kind of stretched circle. • Elliptical galaxies are like spheres that have been pulled to make them the shape of a football. • Stars within elliptical galaxies do not rotate all together but individually around the center.

11. Types of Elliptical Galaxies • Elliptical galaxies are categorized based on how “round” they are on a scale of 0-7. • E0 galaxies are nearly spherical • E7 galaxies are the most elliptical

12. Elliptical Galaxies (cont) • Elliptical galaxies tend to be some of the largest galaxies. • They also contain mostly older, more evolved stars. • The haphazard rotation of stars within an elliptical galaxy suggest that these galaxies formed as a result of interactions between galaxies, such as a collision and merger.

14. Spiral Galaxies • Spiral galaxies are disks of stars all rotating along the same plane around a central point. • Spiral galaxies tend to have pronounced galactic bulges. • Halo has been separated into areas of high and low density. • Areas of high density are called Arms.

16. Types of Spiral Galaxies • Spiral galaxies are categorized based on the tightness and definition of the arms and the size of the galactic bulge. • Sa tightly wound, poorly defined arms, large galactic bulge • Sb intermediate • Sc Open, well defined arms, small galactic bulge.

17. Barred Spiral Galaxies • Most spiral galaxies have a band of stars that extends outward to either side of the core, and are called barred spiral galaxies • Formed from tidal interaction with other galaxies • Have same categorization system as normal spiral galaxies. SBa, SBb, SBc

18. Peculiar Galaxies • Peculiar galaxies develop unusual shapes due to interactions with other galaxies • Ring galaxies • Lenticular galaxies • Irregular galaxies • Dwarf galaxies

19. Ring Galaxies

20. Lenticular • Intermediate between spiral and elliptical galaxies. • Can be barred or un-barred (SB0 and S0)

21. Irregular Galaxies • Some galaxies do not fall into any reasonable category • Irregular-I have some structure • Irregular-II have no structure

22. Dwarf Galaxies • Small galaxies containing up to several billion stars. • Most of the galaxies in the Universe • Can be classified as being elliptical, spiral, or peculiar.

23. Our Galaxy • Milky Way • The word galaxy is derived from the Greek word for milk • Visible from Earth as a band of light in the night sky. • Large Barred Spiral Galaxy

24. How Big? • About 100,000 light years across and about 1000 light years thick • Contains between 200 and 400 billion stars • Has a total mass of more than 600 billion times the mass of our sun. • Difficult to look at as we are inside it.

25. You are here… • It is believed that our galaxy contains 4 major arms and at least two minor arms. • We are located on a smaller arm named Orion’s arm. • 24,000 light years from the center • We take 225-250 million years to orbit the galaxy. • Since humans existed on earth we have traveled less than 1/1000th of that journey.

27. We are not alone… • Galaxies that are near one another are categorized as Groups. • Contain fewer than 50 galaxies • Encompass an area 3-6 million light years in diameter. • Milky Way is part of the Local Group of galaxies • Also contains the Andromeda Galaxy

29. Clusters • Larger than groups, however there is no sharp dividing line. • Contain between 50-1000 galaxies within an area of 6-60 million light years. • We are not part of a cluster of galaxies

30. Superclusters • Largest organization of galaxies • Can span several hundred million light years. • There are approximately 10 million Superclusters with an average of 10,000 galaxies in each. • We are located in the Local Super Cluster or the Virgo Super Cluster