1. Astronomy Stars, Galaxies, and the Universe

2. What is Astronomy?
Astronomy is the study of the moon, stars, and other objects in space
Astronomers study the Universe using telescopes, satellites, probes, as well as manned and unmanned space flights

3. The Universe
Astronomers define the Universe as all of space and everything in it
What’s found in the Universe
Galaxies
Nebulas
Stars
Solar Systems (planets, dwarf planets, moons)
Asteroids, comets, meteors, meteoroids
Dark matter
Does not give off electromagnetic energy and can not be seen directly
Estimated to make up 23% of the Universe’s mass

4. The Origin of the Universe
Astronomers believe that billions of years ago all the matter and energy in the Universe was concentrated into a single hot dense point called a singularity
Tremendous amounts of heat and pressure made this point so unstable that it exploded

5. The Big Bang Theory
According to the Big Bang Theory, the Universe began to expand after an enormous explosion of concentrated matter and energy
As it expanded, the Universe cooled
Atoms formed after a few hundred million years
The first stars and galaxies formed after about 200 million years

6. In Support of the Big Bang
Moving Galaxies-
Edwin Hubble discovered that almost all galaxies are moving away from us and from each other
Hubble’s Law- the farther away a galaxy is, the faster it is moving away from us
Cosmic Background Radiation-
The electromagnetic radiation (thermal energy) leftover from the big bang
Detected with a radio telescope in 1965 by Arno Penzias and Robert Wilson

7. Age of the Universe
Can be determined based on measurements of how fast distant galaxies are moving away from us as and from the cosmic background radiation
Astronomers estimate
that the Universe is
13.7 billion years old

8. Future of the Universe
Background radiation, left over from the Big Bang, has led astronomers to two possible fates for the Universe
Closed Universe- A universe that would expand then collapse back in on itself
Open Universe- A universe that would continue to expand

9. A Closed Universe
A possible future for the Universe in which the force of gravity would pull the galaxies back together
Would result in a “reverse big bang” or a “big crunch”
After many billions of years, all the matter and energy in the Universe would be concentrated in an enormous black hole
The final result could be another “Big Bang” billions of years from now

10. An Open Universe
A possible future where the galaxies continue racing outward, expanding the Universe
Stars would eventually run out of fuel and burn out, leaving the Universe cold and dark
New evidence leads astronomers to believe that dark energy is causing the expansion of the Universe to accelerate and that it will likely expand forever

11. Galaxies A galaxy is a large system of stars and other cosmic bodies
Galaxies are the major features of the Universe
There may be more than 100 billion major galaxies
There are three main types of galaxies

12. Spiral Galaxies
Have a bulge in the middle and arms that spiral outward, like pinwheels
Bright, central nucleus is made up of billions of stars
Spiral arms contain billions of bright, young stars as well as dust and gas
Example: our Milky Way Galaxy
The Milky Way Galaxy is approximately 100,000 light years across. We are located about 25,000 light years from the center, out on one of the spiral arms

13. Elliptical Galaxies
Vary in shape from nearly spherical to flattened disks
Most of the stars are close to the center
Have no arms
Contains billions of stars but little gas or dust
Stars are no longer forming and are generally older than those in the other galaxies

14. Irregular Galaxies Have no definite shape Stars are spread unevenly
Typically smaller than other types of galaxies
Generally have many bright young stars and lots of gas and dust to form new stars

15. Quasars (quasi-stellar objects)
Very bright, distant objects
Many 10 billion light years away
Look almost like stars
Believed to be active young galaxies with enormous amounts of gas revolving around a giant black hole in their centers

16. Stars
There may be as many as 200 quintillion stars in the universe (200,000,000,000,000,000,000
or 2.0 x 10²°)
Stars are huge spheres of glowing gas
Made up mostly of hydrogen
Produce energy by nuclear fusion
Stars differ in size,
mass, color, composition,
temperature, and
brightness

17. Color & Temperature of Stars
The color of stars can be used to determine
their surface temperature
Blue = 35,000°C
White = 10,000°C
Yellow = 6,000°C
Red-orange = 5,000°C
Red = 3,000°C
The temperature at the center
of a star is much greater than
at its surface

18. Stars are divided into 5 main groups according to size
Smallest
Neutron stars- the smallest stars (avg. diameter = 16 km)
White dwarfs- smaller than the Earth (ex. Van Maanen’s star)
Medium-sized stars- vary in size from 1/10 to 10 times the size of the Sun (ex. the Sun & Sirius)
Giant stars- 10 to 100 times as large as the Sun (ex. Aldebara)
Supergiant stars- up to 1000 times larger than the Sun (ex. Rigel & Betelgeuse)
Largest

19. Composition of Stars
Spectroscopes are used to determine the composition of stars
Almost all stars have the same composition
% hydrogen
20 – 30% helium
2% other elements

20. Brightness of Stars
The brightness of a star depends on its size, its surface temperature, and its distance from Earth
Apparent magnitude- the brightness of a star as it appears from Earth
Absolute magnitude- the amount of light a star actually gives off

21. Hertzsprung-Russell Diagram
A chart that shows the relationship between the absolute magnitude and the surface temperature of stars
Astronomers use H-R diagrams to classify stars and to understand how stars change over time
As the absolute magnitude of main sequence stars increases, the temperature increases as well

22. Hertzsprung-Russell Diagram

23. Measuring Distances to Stars
Astronomers use light years to measure distances between stars
A light year is the distance light travels in a year, 9.5 trillion kilometers
Parallax is used to measure distances to nearby stars
Parallax is the apparent change in
position of an object when you look at
it from different places
The star is viewed when Earth is on
one side of the Sun and then six months
later when Earth is on the other side of the Sun

24. Evolution of Stars Stars evolve, or change, over time
The amount of mass a star begins with is the main factor that determines its evolution
The different kinds of stars in the sky represent the various stages in the life cycle of stars

25. Stages in the Life Cycle of Stars
All stars are created from the gases in a nebula
When the contracting gas and dust from a nebula become so dense and hot that nuclear fusion begins, the protostar begins to shine
When a star begins to run out of fuel, its core shrinks and its outer portion expands
The evolutionary path of a star depends on its mass
Medium-sized stars Expand to red giants  white dwarfs  dims to a black dwarf or dead star
Massive stars  Expand to red giants or supergiants  explode in a supernova  become a neutron star or a black hole (depending on initial mass)

26. Possible Evolutionary Paths for Stars

27. Constellations Groups of stars that form a pattern
The revolution of the Earth around the Sun cause different constellations to be seen at different times of the year
Stars located above the north and south poles, called circumpolar stars, appear to move in circles above the horizon each night
Astronomers use constellations as landmarks to locate other objects in the sky

28. Constellations in the Autumn Sky

29. Optical Telescopes
Tools that use large lenses or mirrors to gather rays of light from a star and focus it in one spot
The light-gathering power depends on the area of its lens or mirror (the greater the area, the more light it can gather)
Have three basic functions
Collect more light than the naked eye
Separate distant objects from one another
Magnify images

30. Refracting Telescopes
Use convex lenses to gather light
The lenses
refract (bend)
the rays of
light to form
an image

31. Reflecting Telescopes
Use mirrors to gather and focus light

32. Types of Optical Telescopes

33. How Light Behaves Light travels in straight-line paths called rays
Reflection- When light strikes
a surface, some of it bounces
back
Refraction- The bending of
light due to a change in speed

34. Mirrors Reflect Light
Mirrors are classified based on the shape of their surface
Plane mirrors-
Perfectly flat surfaces
Image appears to be on the other side of the mirror
Concave mirrors-
Surface curves inward
Reflect light rays to the same point in front of the mirror (focal point)
Convex mirrors-
Surface curves outward
Reflected rays spread out from the surface of the mirror

35. Types of Mirrors

36. Lenses Refract Light
Convex lenses- Lenses that are thicker in the center than at the edges
Bend rays of light toward the thicker center
of the lens
The amount of refraction depends on how
much the lens curves (the greater the curve,
the more the light is refracted)
Converge light rays at the focal point
Concave Lenses- Lenses that are thinner in the center than at the edges
Bend rays of light outward toward the
thicker ends of the lens
Diverge rays of light

37. Types of Lenses