The document discusses the life cycles of stars from their birth in molecular clouds to their evolution on the main sequence and eventual death. It begins by explaining how stars are born from collapsing gas and dust in molecular clouds. It then describes how protostars form and evolve into pre-main sequence stars. Next, it discusses how stars exist on the main sequence fusing hydrogen until more massive stars die in supernovae while less massive stars become white dwarfs. The document also provides details on properties of stars like luminosity, temperature, and how the Hertzsprung-Russell diagram is used to study stellar evolution.

1. The Stars of the
Sky and their Life
Cycles
By Drew Sternberg, Brice Millet,
and Anirudh

2. Birth of Stars
2
Star birth is a violent process that produces intense
ultraviolet radiation and shock fronts. Tens of thousands
of stars are born in each cloud of gas and dust.

3. Molecular Cloud & Thermal
Energy
3
The molecular cloud or stellar nursery is the type of
interstellar cloud where stars can be created. In these
clouds there is an abundance of Hydrogen. The cooler
clouds tend to form lower mass stars than the warmer
clouds. Once the cloud contracts it begins to fragment into
smaller and smaller pieces until the pieces are of stellar
mass. Then the density increases causing the temperature
to rise. The fragments then become spinning spheres that
continue to heat up until the internal pressure can support
against gravitational pressure. This is called the
hydrostatic equilibrium. At this point the fragment is now a
protostar.

4. Final Stages of the
Protostar
4
Protostars continue to grow until bipolar jets called
Herbig-Haro objects begin to expel excess mass until
the gas and matter envelope disperses. At this point
protostar becomes a pre-main sequence star. This star’s
energy source is gravitational contraction. It continues
using this until hydrogen begins to fuse in its core. Once
this happens the star is now on the main sequence.

5. Main Sequence Stars
5
Once stars are on the main sequence their lifetime
depends on their size. Massive stars have a much
shorter life span than smaller stars. This is due to
massive stars needing to use more of their “fuel” to
maintain their size and luminosity. Once this fuel of
hydrogen runs out the star enters the end of its life.
Smaller stars go through a planetary nebula and white
dwarf phase, then they will go out and stop glowing
Massive stars explode as a supernova then either
become a neutron star or a black hole depending on how
massive the star is.

6. Alpha Centauri A Star
6
Spectral Type: G2V
Formed approximately 6.5 billion years ago along with
Alpha Centauri B and Proxima Centauri from a large
cloud of gas
Located in the Milky Way Galaxy
Prime example of a Main Sequence Star
Predicted lifespan of about 10 billion years because
of its high temperature and luminosity

7. H-R Diagram
7
Hertzsprung-Russell diagrams or H-R
diagrams plots the temperature of stars
against their luminosity. It also plots the
spectral type with the absolute magnitude
of stars.
This is used to to study stellar evolution.
The real importance of the H-R diagram is
that it is able to describe a star’s internal
structure and evolutionary stage simply via
its position on the diagram

8. Properties of a star
Properties of a star are split into two different
categories:
Observed properties and physical properties
8

9. Physical properties
The four physical properties are:
9
Luminosity
Temperature
Size (Mass/Radius)
Age

10. Observed properties
The main two observed properties are:
10
Color
Color is correlated to the temperature
of the star.
The scale of colors goes as
following(lowest temperature to the
highest):
Red, yellow, white, and blue.
Brightness
Brightness is correlated
with the luminosity of
the star

11. Luminosity vs. Brightness
11
Luminosity
Amount of power a star radiates each
second.
This is an intrinsic property of a star
The unit luminosity is measured in is
watts. Watts = energy per second.
Luminosity is inversely related to size.
The bigger the star the less
luminosity.
Brightness
The amount of energy that we receive
every second per square meter here on
Earth.
The brightness depends on the
distance to the object.
Brightness = Luminosity/Area
Area = 4pi(radius)^2

12. Luminacities and the sun
12
luminosity is the total amount of energy
produced in a star and radiated into space in the
form of E-M radiation.
The Sun radiates 3.9 x 10 33 ergs/sec. Ergs are
units of energy.
The apparent brightness is a measurement of the
amount of photon energy traveling through a
square at any distance from the Earth. The total
area of the sphere with a radius of 1 AU
multiplied by the photon energy per square
whatever is one method of calculating the total
energy release in photons.

13. Inverse Square Law
13
If you double the Luminosity, the Brightness
also doubles.
However, If you double the distance the
Brightness is divided by ¼ due to the
distance being in the denominator and it
being squared in the equation.
L = Luminosity, d = distance to object, &
B = Brightness of Star

14. How to measure Brightness
14
Astronomers use the magnitude
system for the brightness of stars
based on a logarithmic scale. Every 5
magnitude difference is a difference by
factor of 100 in intensity. It’s weird
because the brighter the star the lower
the magnitude. For example, a star
that is -5 magnitude is 100 times
brighter than a star that is 0
magnitude.

15. How to measure Distance
15
Parallax Effect - A shift in viewing
angle that causes an object to appear
to move when compared to background
objects
The distance to the star is inversely
proportional to the parallax angle

16. How to measure Distance 2
16
To measure distance we use a special unit called
a arcsecond (apostrophe looking unit). As you
can see in the diagram an arcsecond is a very
small amount of a circle.
The distance to the star is measured in parsecs.
We find this distance by using this formula:
D = 1/pi(annual parallax angle measured in
arcseconds)
The Closest star to the Earth is the sun, but the
next closest is called the Proxima Centauri.
It’s parallax angle is 0.77” so D = 1/0.77 = 1.33
parsecs away from the Earth

17. 17
1.)https://astronomy.swin.edu.au/cosmos/h/hertzsprung-russell+diagram(slide 7)
2.)https://en.wikipedia.org/wiki/Main_sequence(Slide 5)
3.)https://www.ucolick.org/~bolte/AY4_00/week4/star_properties.html
4.)https://en.wikipedia.org/wiki/Star_formation(Slide 4)
5.)https://www.nasa.gov/content/discoveries-highlights-exploring-the-birth-of-
stars(Slide 2)
6.)https://www.schoolsobservatory.org/learn/astro/stars/cycle(Slide6)
7.)https://en.wikipedia.org/wiki/Molecular_cloud(Slide 3)
8.) https://www.ucolick.org/~bolte/AY4_00/week4/star_distance.html (slide 14,
15, 16)
https://earthsky.org/brightest-stars/alpha-centauri-is-the-nearest-bright-star/
https://osr.org/blog/astronomy/alpha-centauri-a/
Works cited