1. Pearson Prentice Hall Physical Science: Concepts in Action Chapter 26 Exploring the Universe
Chapter 26.2 THE STARS
This presentation is concentrating on stars. We will look at how we measure the distance to stars, chemical and physical characteristics, spectroscopy, how stars produce energy, and finally how stars mass is related to it’s brightness

2. Measuring Distance
Def: a star is a large, glowing ball of gas in space which generates energy through nuclear fusion in its core
Def: a light-year is the unit of distance that light travels in a vacuum in one year
First remember that a star is a large glowing gas in space that creates energy in the core of the star. This is similar to the Sun and as luck would have it that’s because the sun is a star. Many times you will hear that you are seeing light from a star that has already burned out, this is true as stars are so far away. As fast as light does travel it still takes time to reach us here on Earth. Due to these large distances we start measuring them in light years. Remember in class when we were talking about the metric system and scientific notation, we do not want to try to calculate these distances in meters or we’d have really large numbers, even using scientific notation.
So we created a light year. A light year is simply the distance that light travels in space in one year. For fun I looked up the conversion of meters to light years and it is 9.461e+15 meters in one light year. That’s a lot of zeros.

3. PARALLAX
Def: parallax is the apparent change in position of an object with respect to a distant background
Scientists use parallax to measure distances
Hold your thumb at arm’s length in front of you and cover one eye
Keep your thumb in the same spot & cover the other eye
The apparent movement of your thumb is the parallax effect
Astronomers measure the parallax of nearby stars to determine their distance from Earth
Do me a favor. Stick you thumb up near your eyes. Open one eye and line up your thumb with on object on the other side of the room. Now, without moving your thumb open your other eye. What happened? It appeared that the object moved. But did it? No, this is a concept of parallax. Scientist would take measurements of stars location at different parts of the year and see what the apparent shift in motion occurred. This helped to determine how far away stars were from Earth.

4. Chemical and Physical Properties
Astronomers classify stars by:
color, size, and brightness
luminosity
chemical composition and mass
temperature
A star’s color indicates the temperature of its surface
The hottest stars are blue (30,000K) &
cool stars are red (3,000K)
Yellow stars (like our sun) are intermediate temperature (5,000-6,000K)
Stars are classified by color, mass, brightness (which is different from luminosity as luminosity is the amount of light which it gives off, brightness is how much bright it looks), chemical composition, mass, and temperature
The color of the star can tell us many things about the temperature of the star. Really hot stars are blue and white, yellow stars are in the middle (intermediate) temperatures and the red stars are the coolest stars

5. Here are the color ranges, the temperature differences, and some example stars

6. SPECTROSCOPY
Scientists get more precise information about a star by using color spectra & atomic absorption spectra (to indicate the presence of certain elements)
Spectroscopy uses the spectral lines from the electromagnetic spectrum to indicate certain elements. Every element gives off a specific spectra, it’s kind of like a fingerprint. It is unique to that element alone. The absorption lines (dark places) show the areas where the element’s fingerprint.

7. ABSORPTION LINES
Def: absorption lines are a set of dark lines that show where light has been absorbed in the atomic absorption spectra
Each star has its own atomic absorption spectrum which indicates which elements are present in the photosphere
Most stars have a chemical makeup that is similar to the sun
It is mostly hydrogen and helium making up nearly 99% of the star’s mass
Again absorption lines are the dark areas showing where light has been absorbed. This helps us to determine which elements are present in the photosphere, which can tell us what elements are go through fusion in the core. This can help us determine age and when compared with mass then you can also determine what type of star you are looking at.

8. APPARENT BRIGHTNESS
Def: apparent brightness is the brightness of a star as it appears from Earth
Brightness is affected by distance from Earth
Apparent brightness is how bright something appears to be. If you were to take two of your friends out into the woods with flashlights and placed them at great distances away, the one closer to you would APPEAR to be brighter than the one farther away. However, they would both be giving off the same amount of light.

9. ABSOLUTE BRIGHTNESS (luminosity)
Def: absolute brightness is how bright a light REALLY is
Absolute brightness does not depend on distance & it is calculated using distance in the calculation
Absolute brightness is the same as luminosity. It is how much light the star is actually producing. This does not depend on the distance.

10. How to Produce Energy
It wasn’t until the 1960s that scientists discovered that the sun produces its energy in its core central region
The process is nuclear fusion
Recall that nuclear fusion has not happened on Earth yet because of the high temperatures required
The temperatures required for nuclear fusion exist on the sun
The sun produces energy in the core, as we talked about in the previous lecture. The core is where nuclear fusion takes place. The elements are fused together and as a result energy is produced. In average size stars, before they start to die, stars begin fusing hydrogen to helium. This takes place at really high temperatures and these temperatures do not exist here on Earth.

11. MASS AND BRIGHTNESS Astronomers:
can use information about temperature and brightness to estimate diameter (width) and calculate volume of a star
calculate mass indirectly by observing the gravitational interaction of stars that occur in pairs
There appears to be a relationship between absolute brightness and mass
We can determine many things about stars from estimating temperature and luminosity. We can calculate how old it is, what type of star it is, which is helpful in determining how the star will eventually die. The higher the mass of the star the higher the luminosity (absolute brightness) because due to its mass it has a higher rate of fusion giving us more light