7.2 Galaxies

1. 7.2 Galaxies

2. Properties of Galaxies • Contain stars, planets, and dust. • Galaxy that has more dust • Tend to produce more stars because stars form from dust and gases present in nebulae.

3. Properties – Black Hole • Each galaxy contains at least one supermassive black hole at its centre. • A black hole is a region of space where gravity is so strong that nothing, not even light, can escape. • Its gravity is so strong that it can pull a star right into it. • The star is completely destroyed. • The mass of the star adds to the black hole’s original mass, increasing the size of the black hole. • The Milky Way’s black hole has been pulling stars in for at least 7 billion years. • The current size of the black hole has a mass equal to about 3 million stars that are of similar size to our Sun.

4. Properties of Galaxies – Black Hole

5. Properties of Galaxie – Black Hole

6. Properties of Galaxies – Dark Matter • The visible celestial objects might only take less than 10% of the total matter in the space.

7. Properties of Galaxie – Dark Matter • Dark matter – matter in the universe that is invisible because it does not interact with light or any other kind of radiation. It is invisible to direct observation by telescopes. • How did astronomer detect its presence? • Unexpected motion of galaxies • Strong gravitational forces affecting them • The amount of visible mass is not enough. • Example – the stars in the Milky Way revolve around the galaxy’s centre at high speed without being flung off - the dark matter contributes large amount of gravitational force to hold all stars.

8. Properties of Galaxies – Star Clusters • Star clusters – distinct groupings of stars, many stars gather in a relatively small region of space. • Star clusters occurs in two broad types: • Open clusters • Contains a few hundred – a few thousands stars. • Youngest star groups . • Globular clusters • Contains hundreds of thousands of stars. • Drawn together in a spherical form by the star’s gravity. • Oldest star groups.

9. Properties of Galaxies – Star Clusters Open Cluster Globular Cluster

10. Galaxy Shapes • 4 main shapes • Spiral • Barred spiral • Elliptical • Irregular • Measuring Properties • Sizes • Distances • Masses • Luminosities • Dark Matter

11. Galaxy Shapes Elliptical Irregular Spiral

12. Galaxies- Spiral and Barred Spiral • Spiral galaxies are names for the spiral-shaped arms that radiate out from the galaxy’s centre. • The spiral galaxies that appear to have a bar across them are called barred spiral galaxies.

13. Galaxie – Spiral and Barred Sprial • A wave moving outwards from the central regions of the galaxy cause the gas and dust to compress into arm-like bands that rotate around the central hub. • New arms continually form as older ones disappear or change shape. • Gravity keeps the spirals from flying apart. • A typical spiral galaxy completes a full rotation once about every 300 million years. • From the side, a spiral galaxy looks like a thin disk. Most spiral galaxies have hundreds to thousands of star clusters.

14. Galaxies-Spiral and Barred Spiral • Near the core is a widening called the central bulge. • Consisting mainly of old stars • Lack of dusk and gases – new stars rarely form in the bulge • The rest – galatic halo which is made up of individual stars.

15. Spiral Galaxy

16. Galaxies – Elliptical Galaxies • Shape ranges from almost spherical to football-shaped or long and cylindrical (like a pencil) • It is formed when two other galaxies merge. • The largest galaxies in the universe. • Contain very little dust – has fewer young stars.

17. Galaxies – Irregular Galaxies • Irregular shapes • Caused by colliding with another galaxy or got close enough that the gravitational force from the other galaxy drew stars away.

18. Galaxy Cluster • Milky Way is part of a group of about 20 galaxies. • A group of galaxies – galaxy cluster • The Milky Way is in Local Group, which is one group in Local Cluster, which is one cluster in one Local Supercluster..

19. Galaxy Clusters