Astronomy – The study of the origin, composition, and movement of all the objects outside of the Earth system. PLEASE… do not confuse this term with another that sounds close… astrology. Astrology – Using the relative positions of the planets, moon, and stars to predict human events.
Much of what astronomers study of the universe comes to Earth in the form of visible light and other forms of electromagnetic radiation (EMR). There are many different forms of EMR.
Some interesting facts about light: Sometimes light acts as a wave of energy, other times it acts like a particle of energy called a photon . Light travels through space at “light speed” which is approx. 186,000 miles per second. This is equal to approx. 6,000,000,000,000 (six trillion) miles per year. So… 1 light year is equal to six trillion miles. Light years measure distance not time. Light Year – The distance light travels in one year. A unit of measurement used to measure distance in outer space.
Consider this… The universe is so big, we have to measure the distance between other stars, galaxies, etc. using light years. If you could travel at the speed of light, it would take 5.4 hours to reach Pluto. It would take 4.3 years to reach the nearest star outside of our solar system. The fastest space craft ever built travels at about 25,000 m.p.h. At that speed it would take 115,171 years to reach the nearest star outside of our solar system. To travel 1 light year at that same speed would take 26,784 years.
Most of the points of light you see in the sky at night are stars. But, how do we know what stars are made of?
Consider this… Fact #1: Everything in the universe (as far as we know) is made up of the elements found in the Periodic Table of the Elements. Fact #2: All the elements, when heated hot enough, will produce light. Fact #3: You can tell what a distant star is made of just by examining the light it produces. In addition, you can tell how big the star is, how hot it is, how fast it’s moving and in which direction, if there are planets around it, how old it is, how it will die, and much, much more.
Before we continue, lets review some of the basic physics of waves. Wave Height Wave Length
Light travels through space as a wave of energy. When white light enters a prism , it is separated into individual colors based on wave length. Long wavelengths Long wavelengths Short wavelengths RED ORANGE YELLOW GREEN BLUE INDIGO VIOLET How does a prism work? A prism bends light at an angle. Each different color of light is a different wavelength. The prism bends light depending on its wavelength. Different wavelengths of light are bent to slightly different degrees. For example, the angle to which red light is bent is different than orange light, so they come out of the prism in two different locations. This is how the “rainbow” or spectrum is created. Spectrum Spectrum – The “rainbow” of colors produced when white light passes through a prism. Rainbow pic
Solid object Spectroscope – Tool used by astronomers to analyze starlight.
Hydrogen Helium Neon Sodium Mercury Each element of the periodic table, when heated, will produce light. When this light is shown through a prism, it will create a spectrum that is unique for that element. Each element has a “finger print” spectrum that is like no other element.
An actual spectrum of the sun can be very complicated. By matching up the spectral lines for different elements, astronomers have discovered what stars are made of.
How do we know which direction stars are moving, how fast they are moving, and if there are other planets around them?
The Doppler Effect The apparent change in the wavelength of a wave of energy that occurs when an object is moving toward you or away from you. As the car travels toward you, it’s catching up to the sound waves it’s making and the waves are compressed together creating a shorter wavelength. With sound, short wavelengths have a higher pitch. The same effect happens to light waves. Short wavelengths of light are blue in color. As the car travels away from you, it’s traveling away from its own sound waves and stretching them farther apart, creating a longer wavelength. With sound, longer wavelengths have a lower pitch. The same effect happens to light waves. Long wavelengths of light are red in color.
Red Shift – When the entire spectrum of light coming from a star moving away from the Earth appears to be shifted to the red end of the spectrum. Light from star Blue Shift – When the entire spectrum of light coming from a star moving toward the Earth appears to be shifted to the blue end of the spectrum. The farther the shift, the faster the star is moving.
Classifying Stars Stars can be classified in six basic ways: 1. Size 2. Composition 3. Surface Temperature 4. Color 5. Brightness 6. Mass
1. SIZE Stars are grouped into 5 main sizes: Super Giant Star Example: North Star Giant Star Example: Mira Medium Star Example: Sun White Dwarf Star Example: Van Maanen’s Star Neutron Star
The size of objects in space can vary greatly. Here’s an example:
The sizes of objects in space and the distances between them are so enormous, they cannot be put to scale on a textbook page. Any time you've seen a diagram of the solar system, it’s not even close to scale. Distance video wed site
2. COMPOSITION Hydrogen Helium The most common element in stars is hydrogen . The second most common element is helium . Together they make up 96% to 99% of a star’s mass.
3. SURFACE TEMP. and 4. COLOR The surface temp. and color of a star are closely related. The color of a star tells how hot the surface temp is. Coolest Stars Red Approx. 3,000 c Red-orange Approx. 5,000 c Yellow (Sun) Approx. 6,000 c White Approx. 10,000 c Blue or Blue-white Approx. 35,000 c Color Temp . Hottest Stars
5. BRIGHTNESS The brightness of star depends on its temp., size, and distance from Earth. The measure of a star’s brightness is called magnitude. There are two basic types of magnitude: Apparent Magnitude – A stars brightness as it appears from Earth. Absolute Magnitude – The actual amount of light a star gives off.
The actual brightness or, absolute magnitude of the bulb is 100 watts. The closer you are to the light source, the brighter the light will appear to be. This exhibits a higher apparent magnitude . The farther away the light source, the lower the apparent magnitude.
A good example is the North Star also called Polaris. Can you find the North Star in this picture? North Star The reason for the North Star’s small apparent magnitude is because it’s so far away… about 431 light years away. The North Star looks small in the night time sky. However, it is 45 times bigger than the Sun and 2500 times brighter.
North Star Sun To put things in perspective, this is an example of the approximate sizes of the two stars.
6. MASS The main factor that shapes the “life” and “death” of a star is how much mass it began with. The more mass a star starts out with, the shorter it will live. The less mass a star starts out with, the longer it will live. Our Sun is about 5 billion years old and will live for about 10 billion years.
Another way to classify stars is by organizing them according the relationship that exists between brightness and surface temperature. A The Hertzsprung-Russell diagram (H-R diagram) organizes stars according to the relationship between magnitude and surface temp.
Motions of stars Apparent Motion – Because of the rotation of the Earth, the Sun appears to move across the sky. (It rises in the east and sets in the west.) The stars also appear to move across the sky. Because the North Star is above the axis of rotation, all the other stars appear to rotate around it.
Inside a nebula, gas and dust collect together from the force of gravity. Over millions of years, more and more hydrogen gas is pulled together. Collisions between the hydrogen atoms cause the gas cloud to heat up. When the temp inside the cloud reaches approx. 15,000,000 0 C nuclear fusion begins. This causes the formation of a protostar .
Many protostars have disks of gas around them that may form planets.
What is the difference between a chemical reaction and a nuclear reaction? In a chemical reaction, the bonds that hold atoms together are broken to release energy. An example of a chemical reaction is fire, a stick of dynamite, gasoline burning in your car engine, acid dissolving limestone, etc… In chemical reactions, the atoms are not changed. In nuclear reactions , the atoms themselves are broken apart to create new kinds of atoms with the release of energy.
This is an example of a large quantity of dynamite being exploded. It’s a good example of a chemical reaction. This shows the detonation of a small amount of nuclear material. Nuclear reactions release a great deal more energy than chemical reactions.
Inside the core of a typical star, hydrogen atoms are fused together to form helium atoms with the release of a tremendous amount of energy.
The protostar will become a typical star. At this point, its life cycle is fixed and it becomes a main sequence star. How long the star will “live” and how it will “die” depends on how much mass it started with.
The sun is about here in its life cycle. When the Sun is about 10 billion years old, it will expand into a red giant, and then shrink into a white dwarf star.
Inside a star, two opposing forces are at work. The force of nuclear fusion tries to blow the star apart. The force of gravity tries to crush the star inward. When the two opposing forces are equal, the star “burns” at a constant, even rate.
When the Sun begins to run out of hydrogen fuel, it will start to use helium as fuel and create carbon in the process. This will cause the star to increase greatly in size and the surface to cool down. The star becomes a Red Giant . Approx. orbit of the Earth
Once the star runs out of its helium fuel, gravity will crush it down into a tiny white dwarf star. This star does not have enough mass, to create the gravitational forces necessary to fuse carbon atoms and keep the fusion reaction going. The outer shell of hydrogen gas keeps expanding and drifts away to form a planetary nebula . White dwarf star Planetary nebula Red Giant
A white dwarf star is very small, very dense and extremely hot. It may exist this way for billions of years until it cools down to become a black dwarf. Comparison of size of white dwarf using Earth for scale.
The life cycle of a massive star is very similar to a medium-sized star. They continue on the same life-cycle until they become red giant stars. From there, they take a different path. If a star is approx. 6 to 30 times larger than the Sun, nuclear fusion will continue, creating new elements until the element iron is formed in the star’s core.
The star is unable to fuse iron atoms, and the core begins to absorb energy until the star explodes in a super nova .
A massive star called Eta Carinae approx. 8000 light years from Earth is entering its final stages before going super nova. It is expected to explode in the next 10,000 to 20,000 years.
During a super nova explosion, temperatures can reach 100 billion 0 C. At these temps., iron atoms fuse to form new elements. The newly formed elements, along with most of the star’s remaining gases, are blown into space.
A star that began as a massive star will usually end up as a neutron star after the super nova explosion. Inside this small nebula called the Crab Nebula is a neutron star.
A neutron star is about as massive as the sun but only a few miles in diameter so the star is extremely dense. A teaspoon full of neutron star matter would weigh approx. 100 million tons.
Neutron stars spin very fast. As it spins it gives off radio waves. When these radio waves are directed toward the Earth, they are detected as pulses of radio waves. This type of neutron star is called a Pulsar .
If a star begins with a mass that is at least 30 times greater than the Sun, it will explode in a super nova and become a black hole .
One way that astronomers find black holes is to look for the effects they have on the objects around them. In this case, The gravity of a black hole is pulling material off of a companion star and forming an accretion disk around it. As material orbits faster and faster around the black hole, it heats up and sends out a jet of particles into space. Astronomers can locate these jets of particles using different kinds of telescopes.
Another way astronomers locate black holes is by finding stars that are orbiting unseen objects very fast.
A large collection of stars is called a galaxy. This is a photo graph made by the Hubble telescope of deep space. What was once thought to be individual stars turned out to be huge collections of stars.
Astronomers estimate that there are about 200 billion galaxies in the known universe. The galaxy that our sun is in is called the Milky Way Galaxy.
This is an artists conception of what astronomers believe the Milky Way Galaxy looks like.
Some basic Milky Way facts: 1. The Milky Way is a barred-spiral galaxy. 2. The main disk is approx. 100,000 light years in diameter and 1000 light years thick. 3. The Sun orbits the galactic center every 250 million years. 4. The Sun is about 26,000 light years out from the galactic center. Milky Way Galaxy hyperlink 5. Each star in the galaxy is in its own, individual orbit. The spiral arms are only temporary areas where stars have piled up.