Astronomy Stars, Galaxies, and the Universe

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Composition of Stars Spectroscopes are used to determine the composition of stars Almost all stars have the same composition 60 - 80% hydrogen 20 – 30% helium 2% other elements

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

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

Hertzsprung-Russell Diagram

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

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

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)

Possible Evolutionary Paths for Stars

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

Constellations in the Autumn Sky

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

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

Reflecting Telescopes Use mirrors to gather and focus light

Types of Optical Telescopes

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

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

Types of Mirrors

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

Types of Lenses