Stars and Galaxies

1. Stars and Galaxies Galaxies and the Universe

2. Galaxies • We live on the planet Earth, which orbits the Sun. Do the Sun and other stars orbit anything? Where is the Sun in our galaxy? • The Sun is just one star among billions in our galaxy (a large group of stars, gas, and dust held together by gravity).

3. Galaxies • Galaxies are separated by immense distances, often million of light-years. • In the same way that most stars are grouped together in a galaxy, galaxies are grouped together into clusters. • The Milky Way (our galaxy) belongs to a cluster called the Local Group. • The Local Group contains over 30 galaxies, including two large spiral galaxies: the Milky Way and the Andromeda Galaxy .

4. Types of Galaxies • There are three main types of galaxies: • Spiral galaxies • Elliptical galaxies • Irregular galaxies

5. Spiral Galaxies • Spiral galaxies have spiral arms that wind outward from their central regions. • Spiral arms are made up of stars and dust, and in a normal spiral galaxy the arms start close to the center. • Barred spiral galaxies have arms extending from a large bar of stars and gas that passes through the center of the galaxy. • Our Milky Way is a barred spiral galaxy.

6. Spirals and Barred Spirals

7. Elliptical Galaxies • Elliptical galaxies are shaped like large, three-dimensional ellipses. • Many are oval or football-shaped • Others are round or spherical

8. Irregular Galaxies • Irregular galaxies have many different shapes and sizes, and these galaxies do no fit into the other classifications. • Two nearby irregular galaxies called the Clouds of Magellan(Magellanic Clouds) orbit the Milky Waygalaxy

9. Milky Way and Clouds of Magellan

10. The Milky Way • Our home galaxy is a spiral galaxy we call The Milky Way. • It is an average-sized galaxy that contains approximately 200 billion stars, including our Sun. • The visible disk of stars is about 100,000 light-years across and is a few thousand light-years thick.

11. Find the Sun: notice that it is about 30,000 light-years from the center in one of the spiral arms. • All stars orbit around a central region or core. Based on our distance from the center, and at a speed of 235 km/s, the Solar System orbits the center of the Milky Way once ever 240 million years.

12. Normal Spiral or Barred Spiral? • Most older textbooks and diagrams of the Milky Way classify it as a normal spiral galaxy like the nearby Andromeda Galaxy (2.2 million light years away). • There is now evidence that suggests that our Milky Way is actually a barred spiral galaxy. • So what can we see of the Milky Way? Look out at the night sky – all the stars that you see are part of the Milky Way.

13. Origin of the Universe • There have been many different models proposed for how the Universe formed: • Steady State Theory – proposed that the universe has always been the way it is now. It has always existed and always will. As the universe expands new matter is created to keep the overall density of the universe the same or in a steady state. • Oscillating Model – the universe began with expansion occurring in all areas of the universe. Over time the expansion slowed and the matter in the universe contracted. This process happens again and again.

14. Steady state Theory: • Oscillating Model Theory:

15. Expansion of the Universe • Light or sound emitted from an object will shift in color or pitch depending on whether it is moving toward or away from us. • This effect is known as the Doppler Shift. • If a galaxy is moving toward us, its light waves are compressed and are shifted toward the blueend of the spectrum: it is blue-shifted. • If a galaxy is moving away from us, its light waves are stretched out and are shifted toward the red end of the spectrum: it is red-shifted.

17. Red Shift • In 1929, the astronomer Edwin Hubble discovered something important about the light coming from most galaxies. • He observed that when a spectrograph is used to study light from galaxies beyond the Local Group, their light is red-shifted. This indicated that galaxies outside the Local Group are all moving away from us. • He also discovered that the greater the distance of the galaxy, the greater the red shift. • Edwin Hubble discovered that the universe is expanding, and that all galaxies are moving away from each other.

18. The Big Bang Theory • Our current understanding of the origin and expansion of the universe is called The Big Bang Theory. • It states that all matter and energy in the universe existed as an enormously hot and dense point smaller than an atom, and that the cosmos began with an enormous explosion of space and time itself: The “Big Bang.” • Careful measurements of the Hubble red shift indicates that the universe as we know it today began about 13.8 billion years agoand has been expanding ever since. • The universe has been expanding and cooling ever since the Big Bang occurred. The discovery of the cosmic microwave background radiation (CMB) in the 1960s confirms the theory. The background radiation present everywhere in space today shows that the universe has cooled down to about 3 K.