1. Stars

2. Distances To The Stars Stars are separated by vast distances.
Astronomers use units called light years to measure the distance of stars
A light-year is the distance that light travels in a vacuum in one Earth year
Proxima Centauri is the closest star to our sun.

3. Parallax
Astronomers have developed various methods of determining the distance of stars.
The change in position of an object with respect to a distant background is called parallax.
As Earth moves in its orbit, astronomers are able to observe stars from two different positions.
Astronomers measure the parallax of nearby stars to determine their distance from Earth

5. Properties of Stars
Astronomers classify stars by their color, size, and brightness. Other important properties of stars are chemical composition and mass.
Color and Temperature – a star’s color indicates the temperature of its surface.
The hottest stars appear blue.
The cooler stars appear red.
The spectrum of color in a star is from hot to cool is: blues to greens to yellows and reds.

7. Brightness
The brightness of a star as viewed from Earth is dependent on factors such as color intensity and distance.
Apparent Brightness – is the brightness of a star as it appears from Earth.
The apparent brightness of a star decreases as its distance from the Earth increases
Absolute Brightness – is how bright a star really is. The absolute magnitude is how bright a star appears at a standard distance of light years, or 10 parsecs.
Astronomers also measure luminosity — the amount of energy (light) that a star emits from its surface.

8. The Hertzsprung-Russell Diagram
Hertzsprung and Russell had the idea of plotting the luminosity of a star against its spectral type. This works best for a cluster, where you know the stars are all at the same distance. Then apparent brightness vs spectral type is basically the same as luminosity vs temperature. They found that stars only appear in certain parts of the diagram.

10. Composition Each star has its own spectrum.
Most stars have a chemical makeup that is similar to the sun, with hydrogen and helium together making up to 96 to % of a star’s mass.

11. Our Sun’s Composition
The solar composition closely reflects the primordial composition of the universe. It is mostly hydrogen and helium, with a very few other trace elements thrown in:
H: % C: % Fe: % S: %
He: 8.889% Ne: % Mg: % Other: %
O: % N: % Si: %

12. How Stars Form
A nebula is a large cloud of gas and dust spread out over a large volume of space.
Some nebulas are glowing clouds lit from within by bright stars.
A star is formed when a contacting cloud of gas and dust becomes so dense and hot that nuclear fusion begins.

14. Life Cycles of Stars
A star’s mass determines the star’s place on the main sequence (think H-R diagram) and how long it will stay.
The amount of gas & dust available when a star forms determines the mass of each young star.
The larger the star the more energy produced.
Since blue stars burn brightly, they use up their fuel quickly and are short lived.

15. The Death of a Star
The dwindling supply of fuel in a star’s core leads to the star’s death as a white dwarf, neutron star, or black hole.
This image shows the death of a star from NASA’s Chandra X-Ray Observatory

17. Groups of Stars
Astronomers have determined that more than half of all stars are members of star systems.
There are three basic kinds of star clusters: open clusters, associations, and globular clusters.
Astronomers classify galaxies into four main types: spiral, barred-spiral, elliptical, and irregular.
Our Milky Way is a barred-spiral galaxy.

19. Expanding Universe
The observed red shift in the spectra of galaxies shows that the universe is expanding.
Astronomers theorize that the universe came into being in an event called the Big Bang.
Dark matter can only be detected by observing its gravitational effects on visible matter.
Death of a Star: Creation of Dark Matter

20. Artist’s rendition of the beginning of our universe.