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Absolute magnitude
Apparent magnitude
Black hole
Bright nebula
Dark nebula
Emission nebula
Eruptive variables
H – R diagram
Hydrogen burning
Interstellar dust
Main – sequence stars
Neutron star
Planetary nebula
Pulsating variables
Red giant
White dwarf
What is a Star?
A star is a huge sphere of very hot,
glowing gas.
• Stars produce their own light and energy by
a process called nuclear fusion.
• Fusion happens when hydrogen atoms are
forced to become helium atoms.
• When this happens, a tremendous amount of
energy is created causing the star to burn
and shine.
How are Stars Measured?
1. Magnitude
• Absolute magnitude
– The apparent brightness of a star if it were viewed from a
distance of 10 parsecs (32.6 light years)
» A parsec is defined as the distance from the Sun which would
result in a parallax of 1 second of arc as seen from Earth.
» 1 parsec = 3.262 light-years
» Parallax is an apparent displacement or difference in the
apparent position of an object viewed along two different lines
of sight, and is measured by the angle or semi-angle of
inclination between those two lines.
» Astronomers use the principle of parallax to measure
distances to celestial objects including to the Moon, the Sun,
and to stars beyond the Solar System.
– Used to compare the true luminosity or brightness of a star
A simplified illustration of the parallax of an object against a distance background due to a
perspective shift.
When viewed from “Viewpoint A”, the object appears to be in front of the blue square.
When the viewpoint is changed to “Viewpoint B”, the object appears to have moved in front
of the red square.
How are Stars Measured?
• Apparent magnitude
The apparent magnitude
of a celestial body is a
measure of its brightness
as seen by an observer on
Earth, normalized to the
value it would have in the
absence of the atmosphere.
» The brighter the
object appears, the
lower the value of its
Visible to
human eye
to Vega
Number of
brighter than
1 602
4 800
14 000
42 000
121 000
340 000
How are Stars Measured?
• Luminosity, temperature, and size
– Hertzsprung-Russel Diagram
» a graphical tool that astronomers use to classify stars
according to their luminosity, spectral type, color,
temperature and evolutionary stage.
– Stars in the stable phase of hydrogen - burning lie along the
Main Sequence according to their mass.
– After a star uses up all the hydrogen in its core, it leaves the
main sequence and moves towards the red giant branch.
The most massive stars may also become red supergiants,
in the upper right corner of the diagram.
The lower left corner is reserved for the white dwarfs.
Main Sequence Stars
The main sequence is the point in a star's evolution during which it
maintains a stable nuclear reaction.
It is this stage during which a star will spend most of its life.
Our Sun is a main sequence star.
A main sequence star will experience only small fluctuations in
luminosity and temperature.
The amount of time a star spends in this phase depends on its mass.
Large, massive stars will have a short main sequence stage while
less massive stars will remain in main sequence much longer.
Very massive stars will exhaust their fuel in only a few hundred
million years.
Smaller stars, like the Sun, will burn for several billion years during
their main sequence stage.
Very massive stars will become blue giants during their main
Main Sequence Stars
• Core temperature > about 10
million K hot enough that
hydrogen nuclei (protons)
can overcome fuse together.
• Through several steps,
hydrogen is fused to form
helium nuclei. (hydrogen
• In the process a small
amount of mass is converted
into energy, released in the
form of high-energy gamma
• This hydrogen fusion
provides the radiation
pressure that supports main
sequence stars against
further gravitational collapse.
In the Sun
On Earth
Main Sequence Stars
The main factor that determines
where a star lays on the main
sequence is its mass.
A mass = 1/10 that of the Sun 
enough gravitational force to heat
the core to 10 million K (hydrogen
less massive = no fusion
brown dwarf or a "failed" star forms
Emits only infrared radiation
Greater mass = higher core
temperature = greater rate of
hydrogen fusion.
Higher-mass stars
Comes at a cost.
more energy than lower mass ones
more luminous than lower mass ones.
High mass stars consume their core
hydrogen fuel much faster than lower-mass
Our Sun has sufficient hydrogen in
its core to last about 10 billion years
on the main sequence.
A 5 solar-mass star would consume its core
hydrogen in about 70 million years
An extremely massive star may only last 3
or 4 million years.
Key Properties of Main Sequence Stars
Main sequence
lifespan (yrs)
Key Properties of Main Sequence Stars
Spectral Class Type of Stars
1. Class O
Main sequence star
Hydrogen – fusing
Very hot and extremely luminous
Have between 15 and 90 times the mass of the Sun
Surface temperatures between 30,000 and 52,000 K
Between 30,000 and 1,000,000 times as luminous as the
Bluish in color
Output is in the ultraviolet range
Very hot cores
Burn through hydrogen very quickly
First stars to leave the main sequence
Over a million times our Sun's output
Rarest of all main-sequence stars
Most massive
Example: Zeta Orionis
• Class B
Main sequence star
Hydrogen – fusing
Very luminous and blue
Very hot and powerful
Surface temperatures
between 10,000 and
30,000 K
» Short lived
• Cluster together in what are called OB associations
» associated with giant molecular clouds.
• Example: Pleiades
• Class A
Main sequence star
Hydrogen – fusing
more common naked eye stars
white or bluish-white
surface temperatures
between 7,600 and 10,000 K
• Example: Vega
Class F
• Main sequence star
• Hydrogen – fusing
• surface temperatures between 6,000 and
7,600 K
• Yellow – white color
• Example: Procyon
Class G
Best known
Main sequence star
Hydrogen – fusing
Surface temperature of between 5,300 and 6,000 K
Converts H to He in its core by means of nuclear
• Yellow to white in color
• Example: Sol (our sun)
Class K
• Main sequence star
• Hydrogen – fusing
• Orange color
» Giants and supergiants
• Surface temperatures between 3,900 and 5,200 K
• Are of particular interest in the search for
extraterrestrial life
» They are stable on the main sequence for a very long
time (15 to 30 billion years, compared to 10 billion for
the Sun).
» May create an opportunity for life to evolve on
terrestrial planets orbiting such stars.
• Example: Epsilon Eridani
Class M
• Most common
• Late Main sequence
» Red giants and red dwarfs
• Red Dwarf
Small and relatively cool star
Surface temperature of less than 4,000 K
Most common star type in the galaxy
Less than half the mass of the Sun (down to
about 0.075 solar masses)
• Example: Barnard’s Star
• Red giants
» Evolve from main sequence stars
» Low to intermediate mass (0.5 – 10 solar
» Late phase of stellar evolution
» Outer atmosphere is inflated and tenuous
» Immense radius and low surface temperature,
(5,000 K and lower)
» Hydrogen fusing shells
» Tens to hundreds of times larger than that of the
» Example: Aldebaran
Red Giants
• A red giant is a large star that is reddish or orange in color.
• It represents the late phase of development in a star's life, when its
supply hydrogen has been exhausted and helium is being fused.
• This causes the star to collapse, raising the temperature in the
• The outer surface of the star expands and cools, giving it a reddish
• Red giants are very large, reaching sizes of over 100 times the
star's original size.
• Very large stars will form what are called red supergiants.
• Betelgeuse in Orion is an example of a red supergiant star.
Brown Dwarf
• A failed star
• Some protostars never reach the critical mass required
to ignite the fires of nuclear fusion.
• If the protostar's mass is only about 1/10 that of the Sun,
it will glow only briefly until its energy dies out.
• What remains is a giant ball of gas that is too massive to
be a planet but not massive enough to be a star.
• They are smaller than the Sun but several times larger
than the planet Jupiter
• They emit no light or heat.
• Just energy in the infrared wavelength
Gliese 229B
• The best known brown dwarf, and one that we can actually look at
through an Earth-bound 60-inch telescope, is Gliese 229B, discovered
in 1995. This one is in a binary system with the low-mass red dwarf
Gliese 229A, at a distance of just 19 light-years from the Sun.
• The separation between the brown dwarf and its companion star is
about the same as that between the Sun and Pluto. Its luminosity is
about one tenth of the faintest star. Its spectrum has large amounts
of methane and water vapor. Methane could not exist if the
surface temperature were above 1500K.
• Astronomers consider its temperature to be
about 900K (compared to Jupiter’s 130K), its
mass to be between 20 and 55 Jupiters, and
the age of the binary system to be between 1
and 5 billion years old. It has a smoggy haze
layer deep in its atmosphere, essentially making
it "much fainter in visible light than it would
otherwise be".
Binary Stars
A system of two stars that are
gravitationally bound to each other.
They orbit around a common point,
called the center of mass.
It is estimated that about half of all
the stars in our galaxy are part of a
binary system.
• Visual binaries can be seen as two separate stars through a
• Spectroscopic binaries appear as one star and can only be detected
by studying the Doppler shifts on the star's spectrum.
• Eclipsing binaries are binary systems where one star blocks the light
from another as it orbits its companion.
Types of Binary Systems
3 Types
Detached binary: neither star fills
its Roche lobe, so that there is no
significant mass transfer between
the components.
Semidetached binary: one of the
stars fills its Roche lobe, which
results in this star losing material in
a matter stream that either falls
directly onto its companion, or, as
is more usual, that enters an
accretion disk.
Contact binary both components
fill their Roche lobes or, more
often, overflow them so that there
is a common convective envelope.
Roche lobe
The volume around a star in a binary system in which, if you were to release a
particle, it would fall back onto the surface of that star.
A particle released above the Roche lobe of either star will, in general, occupy
the circumbinary region that surrounds both stars.
The point at which the Roche lobes of the two stars touch is called the inner
Lagrangian point.
If a star in a close binary system evolves to the point at which it fills its Roche
lobe, calculations predict that material from this star will overflow both onto the
companion star (via the L1 point) and into the environment around the
binary system.
Cepheid Variable Stars
• Stars that changes in brightness.
• These fluctuations can range from seconds to years depending on
the type of variable star.
• Stars usually change their brightness when they are young and
when they are old and dying.
• They are classified as either intrinsic or extrinsic.
• Intrinsic variables change their brightness because of conditions
within the stars themselves.
• Extrinsic variables
change brightness
because of some
external factor,
like an orbiting
companion star.
These are also known
as eclipsing binaries.
White Dwarf
• A small star composed mostly of electrondegenerate matter.
• weight of overlying material tries to force all of the
electrons surrounding the atomic nucleus into the
lowest energy state. The electrons resist and exert
a pressure that halts further collapse.
• Very dense;
• Comparable to that of the Sun and its
volume is comparable to that of the Earth.
After the hydrogen–fusing lifetime of a main-sequence star of low or
medium mass ends, it will expand to a red giant which fuses helium to
carbon and oxygen in its core.
If a red giant has insufficient mass to generate the core temperatures
required to fuse carbon, around 1 billion K, an inert mass of carbon and
oxygen will build up at its center.
After shedding its outer layers to form a planetary nebula, it will leave
behind this core, which forms the remnant white dwarf.
Usually, therefore, white dwarfs are composed of carbon and oxygen.
The material in a white dwarf no longer undergoes fusion reactions, so the
star has no source of energy, nor is it supported by the heat generated by
fusion against gravitational collapse.
It is supported only by electron degeneracy pressure, causing it to be
extremely dense
Size Comparison of White
Dwarf to Earth
Neutron Star
• A type of stellar remnant that can result from the gravitational
collapse of a massive star during a supernova event. Such stars
are composed almost entirely of neutrons.
• Neutron stars are very hot and are supported against further
collapse by electron degeneracy pressure.
– No two subatomic particles can occupy the same place
• A typical neutron star has a
mass between 1.35 and about
2.0 solar masses
• A radius of about 12 km
• Density is 2.6×1014 to 4.1×1014
times the density of the Sun
• A pulsar is a neutron star that
emits beams of radiation that
sweep through Earth's line of
• A rotating neutron star that emits beams of radiation
that sweep through Earth's line of sight
• The "pulses" of high-energy radiation we see from a
pulsar are due to a misalignment of the neutron star's
rotation axis and its magnetic axis.
• Pulsars seem to pulse from our perspective because the
rotation of the neutron star causes the beam of radiation
generated within the magnetic field to sweep in and out
of our line of sight with a regular period, somewhat
like the beam of light from a lighthouse.
• The stream of light is, in reality, continuous, but to a
distant observer, it seems to wink on and off at regular
Black Hole
• Black holes are remnants of stellar masses that form
when heavy stars (> 10 solar masses) collapse in a
supernova at the end of their life cycle.
• Creating a region of spacetime from which nothing, not
even light, can escape.
– Due to extreme gravitational forces
• 2 Types
– Stellar: formed from collapsed massive star – about 30 km in diameter
– Supermassive : found in galactic cores - about 10 AU
Black Hole Anatomy
Black Hole Structure
• A black hole's entire mass is concentrated in an
almost infinitely small and dense point called a
• This point is surrounded by the event horizon the distance from the singularity at which its
escape velocity exceeds the speed of light.
• A rotating black hole is surrounded by the
ergosphere, a region in which the black hole
drags space itself.
Black Hole Structure
The singularity forms when matter is
compressed so tightly that no other force of
nature can balance it. In a "normal" star, like
the Sun, the inward pull of gravity is
balanced by the outward pressure of the
nuclear reactions in its core. In the collapsed
stars known as white dwarfs or neutron
stars, other forces prevent the ultimate
If there is too much mass in a given volume,
though, the object reaches a critical density
where nothing can prevent its ultimate
collapse to form a black hole.
Because gravity overcomes the other forces
of nature, a singularity follows its own
bizarre rules of physics. Time and space as
we know them are crushed out of existence,
and gravity becomes infinitely strong.
As the distance from the singularity
increases, the escape velocity decreases.
Escape velocity is the speed at which an
object must move to get away. For Earth,
the escape velocity is around seven miles
(11 km) per second. In other words, a
spacecraft must go at least that fast to
escape Earth's gravitational pull and travel
to another planet.
At a certain distance from the singularity, the
escape velocity drops to the speed of light
(about 186,000 miles/300,000 km per
second). This distance is known as the
Schwarzschild radius, in honor of Karl
Schwarzschild, who first defined it. This
radius depends on the mass of the black
hole. For a black hole as massive as the
Sun, the radius is about two miles (3 km).
For every extra solar mass, the radius
increases by two miles.
Black Hole Structure
This radius enfolds the singularity in a zone of blackness - in other words, it
makes a black hole black. It gives the black hole a visible surface, which is
known as the event horizon. This is not a solid surface, though. It is simply
the "point of no return" for anything that approaches the black hole. Once
any object - from a starship to a particle of light - crosses inside this horizon,
it cannot get back out. It is trapped inside the black hole.
Anything that enters the black hole increases its mass. And as the mass
goes up, the size of the event horizon gets bigger, too. So if you feed a
black hole, it gets fatter!
If the black hole doesn't rotate, then its gravitational influence on its
environment is straightforward. If the black hole is spinning, though, then its
gravitational effects are more complicated. It actually pulls the fabric of
spacetime along with it - an effect called frame dragging. This area is known
as the ergosphere. Seen in cross-section, it is oval-shaped, with the region
of influence extending farther into space at the black hole's equator than at
its poles.
Stellar Black Hole
Supermassive Black Hole