1. Stars

2. How does a Star Glow? All stars get their energy from FUSION
Fusion – combining of the nuclei of lighter elements (hydrogen) to form a heavier element (helium).
Fusion occurs in the core (middle) of a star.

3. Brightness Term #1 Apparent Magnitude
Measure of how bright a star appears to be to an observer on Earth.
A low magnitude number means that the star is bright.
The dimmest stars that can be seen with our eye have a magnitude 6.
The Sun has a magnitude of -27, by far the brightest object in the sky.

4. Brightness Term #2 Luminosity
actual brightness of a star and depends on its surface area (size) and temperature.
If two stars had the same temperature but different sizes; the larger star would have a greater luminosity.
If two stars were the same size but had different temperatures, the hotter star would be more luminous.

5. Brightness Term #3 Absolute Magnitude
Measure of how bright the star would be if all stars were at the same distance from Earth.
If you are moving the star closer to earth, it will be brighter.
If you are moving the star farther from earth, it will be dimmer.

6. Composition of Stars All stars consist mostly of hydrogen and helium
No two stars contain exactly the same elements in the same amounts.
Astronomers use spectral analysis from a spectroscope to determine a star’s composition.

7. Mass And Size Of Star
A star’s mass is something that we cannot observe directly.
We can calculate what a stars mass might be on the basis of other observations.
it’s properties
Gravitational influence on other objects.
Stellar Masses
The mass of all stars is expressed as multiples of the mass of the sun.
Sun’s mass is 1 solar mass.
Size
Stars vary more in size than in mass.
Smallest stars are smaller than Earth.
Largest star known is 2000 times larger than our sun.

8. Color of a Star
Range of color a star emits depends on its surface temperature.
Surface Temperature (C)
Color
Prominent Elements in Spectrum
Above 30,000
Blue
Ionized Helium
,000
Bluish white
Neutral Helium
White
Metals, Hydrogen
Yellow White
Yellow
Orange
Below 3900
Red
Titanium Oxide

9. Life Cycle of a Star A Star is Born First…..
All stars begin in a nebula.
A nebula is a large cloud of gas and dust.
Gravity pulls the gas and dust together to form a protostar.
Protostar is the earliest stage of a star’s life.
Then….
The contracting gas and dust become so hot that nuclear fusion starts
Once fusion starts a star is “born.”
Most stars become part of the main sequence at this point.

10. Death of a Small Star Death of all Stars White dwarf characteristics
A star begins to die when it runs out of fuel (Hydrogen)
Center of star shrinks and the outer part of the star expands becoming a giant
What death the star will have depends on the mass of the star
Small Stars
Small and medium sized stars end up as white dwarfs.
Outer layers keep getting bigger and eventually drift into space.
The blue-white hot core of the star is left.
The core is called a white dwarf.
Planetary nebula: glowing gas around a white dwarf.
White dwarf characteristics
size of Earth and massive as the sun
1 million times as dense as the sun.
When a white dwarf stops glowing it is dead, and called a black dwarf.

11. Death of a Medium Star Dying giant star explodes.
The explosion is called a supernova
Material from the star can either create a new nebula to form new stars or form neutron stars.
Neutron Characteristics
Smaller and denser than white dwarfs.
3 times the mass of the sun
20 km in diameter (same size as a town here)
PULSAR- spinning neutron star.

12. Death of a Large Star
The supergiant explodes in a supernova explosion.
This leaves behind a black hole.
Characteristics of a Black hole
Nothing can escape from a black hole.
We can detect black holes by
X-rays coming from the hot gas going into the black hole.
Effect of the black holes gravity on a nearby star.

13. Length of a Star’s Life How long a star lives depends on its mass.
Small stars use up their Hydrogen slower than large stars.
Small stars can live for up to 200 billion years.
Medium stars (sun) live for about 10 billion years.
Larger stars can live for a few million years.

14. Life Cycles of Low Mass Stars
© Sea & Sky
We have already discussed the evolution of a low-mass star in the previous session. We will now discuss how massive stars cook elements in their cores.

15. Life Cycles of Massive Stars
© Sea & Sky
We have already discussed the evolution of a low-mass star in the previous session. We will now discuss how massive stars cook elements in their cores.

16. Activity: Lifecycle of a Star

17. H-R Diagram Hertzsprung-Russell Diagram
Graph used to find out if temperature and brightness of a star are related.
Temperature plotted on the X-axis
Brightness plotted on the Y-axis
Most stars form a diagonal band called the main sequence.
Surface temperature increases as brightness increases.
90% of stars are main-sequence stars

19. Activity: Classifying Stars!!

20. How Do Stars Make Elements?
Sun generates energy in its core by “cooking” hydrogen to form helium.
This is called nucleosynthesis.
Let us recap how stars make their energy.
A star such as the Sun generates its energy in its core. The energy is generated by a process known as nuclear fusion. Think of nuclear fusion as a form of cooking… the core of the Sun is very hot (15 million degrees celsius), and the hydrogen is “cooked” and forms helium.

21. Nucleosynthesis Protons and Neutrons are conserved.
They fuse to form new elements.
Requires and releases immense heat

22. Nuclear Fusion The “cooking” of elements is called nuclear fusion
During nuclear fusion, two or more atoms of one element combine to form one atom of a different element
In nuclear fusion, two or more atoms (or more precisely the nuclei of those atoms) merge together to form a new atom (nucleus).
The precise processes are a little complicated but 4 hydrogen atoms need to fuse together to form helium. Each second, the Sun converts 600 million tons of hydrogen into helium.

23. Conservation of Protons and Neutrons
Protons and Neutrons can not be created nor destroyed
It is an addition problem!

24. Time to Practice!!!

25. Elements in the Universe
Hydrogen and some helium was made at the beginning of the Universe (Big Bang).
All other elements were made inside of stars, and then spewed out into space by the supernova explosions!
Low mass stars up to carbon
High mass stars up to iron
What about elements with atoms heavier than iron? Such as Uranium, Gold, and so on?
The heavy atoms are made during the supernova explosion itself!
There is so much energy during the explosion that iron atoms can be forced together to form larger atoms.
Supernova are the source of most elements in the Universe…
Hydrogen and (some of the) helium have been around since the beginning of the Universe. But all of the other elements that we see around us were made inside of stars and spewed into space in supernova explosions.
What about elements heavier than iron such as Uranium, Silver or Gold? They are made during the supernova explosion itself when the temperature gets even hotter than in the core of the star!

26. Interior of a Massive Star
Just before a supernova, the inside of the star has shells of various elements.
Just before the explosion, the star has an “onion shell” structure… iron in the core, surrounded by shells of silicon, magnesium, neon, oxygen, carbon, helium and (on the outside) hydrogen.

27. Supernova Remnants
X-ray picture of the “Cas-A” supernova remnant. The elements in this gas will eventually be dispersed into space, maybe to form new stars, planets and people!
We actually see elements in the debris of old supernovae (known as supernova remnants). This is an X-ray picture of the supernova remnant Cassiopeia-A taken with the Chandra X-ray Observatory. This expanding cloud of very hot gas is loaded with elements that will eventually be distributed through space.

28. Every substance gives off light when it gets hot enough.
Using the Electromagnetic Spectrum to determine a star’s composition, motion and distance to Earth
Every substance gives off light when it gets hot enough.
Each element gives off its own special color. (emission spectrum)
We use spectra to determine which elements make up stars.
Motion and distance is determined by the amount of red shift.
Red shift = moving away
The farther the star the greater the red shift (Hubble’s Law)