A barred spiral galaxy is a rotating disk of stars, gas and dust with a central bar of stars. Stars are classified based on their surface temperature, from hottest O-type blue stars to coolest M-type red stars. Stars evolve over their lifetimes, starting as protostars and becoming main sequence stars, then red giants, white dwarfs, neutron stars or black holes at the end of their lives.

2. Scientists believe
that our galaxy is a
barred spiral galaxy
A barred spiral
galaxy is a
rotating, flat disk of
stars, gas and dust
with a bar-shaped
center made of
stars.

3. A STAR IS BORN
 Protostar
 Every star starts out in a mass of clouds and
gas called a nebula.
 causes hydrogen gas in the nebula to spin.
 As the hydrogen gas spins, it becomes
hotter. It begins to glow.

4. Medium and larger protostars spin until they
reach extremely hot temperatures.
Through the process of nuclear fusion, hydrogen
in the stars begins to turn into helium.
This is the same process that goes on in the
Sun, and makes it able to give light and
heat to Earth.

5. STAR COLORS
 A star’s color comes from its surface
temperature.
 How does one star come to have a hotter
surface temperature than another?
 It’s a matter of size.
 The more mass a star has, the hotter the
surface temperature.

6. STARS ARE CLASSIFIED BY THEIR SURFACE
TEMPERATURE.
 that is associated to specific spectral
patterns
 the spectral classification includes 7
main types: O, B, A, F, G, K, M.
 A popular mnemonic for remembering
this order is "Oh, Be A Fine Girl, Kiss
Me".

7. MORGAN-KEENAN SPECTRAL CLASSIFICATION
Class Temperature Star Color
O 30,000 - 60,000 °K Blue
B 10,000 - 30,000 °K Blue
A 7,500 - 10,000 °K White
F 6,000 - 7,500 °K White (yellowish)
G 5,000 - 6,000 °K Yellow (like the Sun)
K 3,500 - 5,000 °K Orange
M 2,000 - 3,500 °K Red

8. WHAT IS A NOVA?
 . A nova is an explosion that happens on a
white dwarf, which is a very old star.

9. This swirl of colors
is made of material
left from Kepler’s
Supernova located
in the Milky Way
Galaxy
Kepler’s
Supernova was
the last
supernova seen
in our galaxy.

10. A SUPERNOVA GETS AT THE CORE
 A supernova is like a nova, except the
explosion takes place at the core of the star,
not the surface.
 Every supernova and nova is important,
because each explosion creates dust
 This dust forms the building blocks for
nebulae, where new stars are born.

11. BLACK HOLES
 After a supernova, a star may leave behind a
burned part
 This part can collapse and absorb the light
around it.
 This is a black hole, the end of a giant star.
 A star must be 10 to 15 times larger than the
Sun in order to end as a black hole.
 A smaller star will simply cool rather than
collapsing.

12. CLUSTERS OF STARS
Open clusters
have mostly young,
bright, blue stars that
were born together.
They usually have irregular shapes.
At 400 light years away, the Pleiades is Earth’s
closest open cluster.
Example is Pleiades

13. GLOBULAR  can have as many as a
CLUSTERS
million stars – more than
open clusters.
 have a sphere-like shape,
with many stars at the
center.
 A globular cluster named
M15 is the closest of its
kind to Earth.

14. TYPES OF STARS
 Protostar
a collection of gas that has collapsed down
from a giant molecular cloud.
 Over time, gravity and pressure increase,
forcing the protostar to collapse down.
 All of the energy release by the protostar
comes only from the heating caused by the
gravitational energy – nuclear fusion
reactions haven’t started yet.

15. T TAURI STAR
 stage in a star’s formation and evolution right
before it becomes a main sequence star.
 T Tauri stars don’t have enough pressure and
temperature at their cores to generate
nuclear fusion
 they do resemble main sequence stars
 Stars will remain in the T Tauri stage for
about 100 million years.

16. MAIN SEQUENCE STAR
 Our Sun is a main sequence star, and so are
our nearest neighbors, Sirius and Alpha
Centauri A
 converting hydrogen into helium in their
cores, releasing a tremendous amount of
energy

17. RED GIANT STAR
 A shell of hydrogen around the core ignites
continuing the life of the star, but causes it to
increase in size dramatically.
 star has become a red giant star, and can be
100 times larger than it was in its main
sequence phase.
 The red giant phase of a star’s life will only
last a few hundred million years before it
runs out of fuel completely and becomes a
white dwarf.

18. WHITE DWARF STAR
 When a star has completely run out of
hydrogen fuel in its core and it lacks the
mass to force higher elements into fusion
reaction, it becomes a white dwarf star
 A white dwarf shines because it was a hot
star once, but there’s no fusion reactions
happening any more

19. A white dwarf will just cool down until it
because the background temperature of the
Universe.
This process will take hundreds of billions
of years, so no white dwarfs have actually
cooled down that far yet.

20. RED DWARF STAR
 Red dwarf stars are the most common kind
of stars in the Universe.
 Red dwarf stars are able to keep the
hydrogen fuel mixing into their core, and so
they can conserve their fuel for much longer
than other stars.
 red dwarf stars will burn for up to 10 trillion
years.

21. NEUTRON STARS
 If a star has between 1.35 and 2.1 times the
mass of the Sun, it doesn’t form a white
dwarf when it dies.
 Instead, the star dies in a catastrophic
supernova explosion, and the remaining core
becomes a neutron star.

22. SUPERGIANT STARS
 The largest stars in the Universe are
supergiant stars.
 supergiants are consuming hydrogen fuel at
an enormous rate and will consume all the
fuel in their cores within just a few million
years.
 Supergiant stars live fast and die young