Unit 9: The lives of stars from birth through middle ages Flashcards
What do stars condense from
Interstellar gas and dust
What is the interstellar gas composition (in mass percentage)
74% - hydrogen
24%- helium
others 1%
How many chemical elements have been discovered
150
What is the origin or the universe
The big bang model
What was the first chemical element and how was it created
Hydrogen
The big bang theory
How are first generation stars made
various hydrogen atoms stick together due to gravity and form first generation stars
Are the stars we see in the sky alive or dead
Most of them are dead
What is the death of a star
- The star sheds the remaining unused hydrogen into space instead of converting it into helium
- After shedding off the hydrogen creates clouds and they begin to condense and form the second generation star
Is the sun a first or second generation star
second generation because of its age
What is interstellar reddening
- Blue light scatters off dust in interstellar clouds, leaving the light we see from the stars redder than it would otherwise appear.
Due to the reflected scattered blue light we see the dust cloud blue
Why is the sky blue?
sky appears blue because the scattered hydrogen is more blue than red
What colour does the sun appear
More yellow and green than it really is
How do we reveal regions of new stars formation
Clouds of interstellar dust glow
Clouds of hydrogen = red
Clouds of oxygen and hydrogen = red middle, blue ring around it
What are clouds of interstellar dust?
regions in space where second generation stars are being formed
Which constellations/regions are sites of intense CO emission and what does it mean
Region of orion and horsehead nebulae
This indicates that stars are forming in these regions.
The glowing gases in emission nebulae are excited by _____ from _________?
Ultraviolet radiation from young massive stars
- Excited atoms absorb UV photons and omit photons of visible light
- New hot stars emit UV photons
What is the horse head nebula
Dark nebula
ionized hydrogen in the constellation Orion
- Hot hydrogen rich cloud glowing red
- cold dark gas and dust
What is necessary for star formation?
Jeans instability (pressure and gravity)
What is Jeans Instability?
Jeans instability causes the collapse of interstellar gas clouds
- Sufficient density of gas surrounding dark core
What does it mean when the dark region developed a jean instability?
the density increased beyond the jeans limit
- low temp
In jeans instability the central gases are heating as they fall into _________
the newly forming protostar
- kinetic energy of molecules transforms into heat
What happens as the mass of a protostar increases?
the temperature also increases (it surface gets brighter while its core heats up more and more)
What do low temperatures do to molecules?
low molecular speed
What is a pre-main-sequence star?
Once little gas is left in the centre of the dark core and its accretion stops, the object becomes a pre-main-sequence star
How long does it take for a protostar to grow into a pre-main-sequence star
10^5 years
What does the protostar form from?
interstellar gas and dust
What is the size of a protostar compared to the Sun
a protostar of 1M is about 5 times larger in diameter than sun
Does a pre-main-sequence star contract slowly or rapidly
slowly
unlike the rapid collapse of a protostar
At what temperature does the core of a pre-main-sequence star need to reach for hydrogen fusion to happen
10^7K
- the energy emission makes pressure inside the star sufficient to halt in contraction. The balance between gravity and internal pressure defines the star’s diameter
What does the pre-main-sequence star evolution depend on and how long does it take
the initial mass
- more massive stars just skip the pre-main-sequence star stage
- few 10s million years for a sun like star to about 10^5 years for a 5M star
What is different about the core of pre-main-sequence stars
there is no thermonuclear reactions
What is the lower star mass limit
0.08M
What are brown dwarfs
lighter bodied stars, it is impossible for fusion reactions to happen, meaning it will be a failed star
What is the pistol
the brightest know star that formed nearly 3 million years ago and originally had 100-200 M
What are main-sequence stars
stars with nuclear reactions fusing hydrogen into helium at their cores at a constant rate
(stars spend most of their life cycles on the main sequence)
Explain main sequence lifetimes
larger the Mass in M the brighter and hotter it is, though the bigger it is the less time spent on the main sequence
What is the mass of Red Dwarfs
0.08M - <0.4M
What kind of fusion happens in red dwarfs core and how long do they stay on the main sequence
- hydrogen fusion
- remains on the main sequences for hundreds of billions of years, none of them have yet left the main sequence
Do Red Dwarfs have thermonuclear reactions after the main sequence?
No
What is a Red Dwarfs terminating State
Helium body cooling t very low rate, moving down and the the right from the main sequence
Why are Red Dwarfs living so long?
- Low rate of thermonuclear fusion (4H - He)
- They are completely convective
They have a permanent supply of fresh hydrogen to helium thermonuclear core
What Is the mass of low mass stars
0.4M < 8M
How long do low mass stars stay on the main-sequence line
15 billion years to 1 billion years
Do low mass stars have thermonuclear reactions after the main sequence
yes
4He + 4 He + 4 He ——> 12C + Y
12C + 4He —–> 16 O + Y
in a rich helium core, hydrogen fusion in a she
What is a red giant
Hydrogen fusion in a shell
in rich helium core
What is the terminal state of low mass stars
white dwarf- carbon oxygen body cooling at very low rate
- white drwafs occupy the centre of the H-R diagram
Do red giants have thermonuclear reactions
no
-the outer layers of the red giants are hydrogen burning shells with a helium core (in main sequence star the hydrogen burning is in the core)
What kind of star will the sun turn into
white dwarf?
we have carbon oxygen core, then helium fusion shell, than hydrogen fusing shell
What rate does the luminosity of our sun increase at
10% per 1 billion years
What is the life cycle of the sun
(0) - Birth
(4. 5 billion years)- now
(5. 5- 8.5)- gradual warming
(10) - red giant
(11) - planetary nebula
(14) - white dwarf
What is the low mass star final
- a planetary nebula and a white dwarf in its centre
What is the matter inside a white dwarf called
degenerate state
What is the average size of a white dwarf, density, and mass
about the size of the earth
- density about 10^9 kg/m3
- mass of one teaspoon of this matter is about 5 tons
What kinda of star is sirius
white dwarf
very young and bright
-brightest star
What is a nova star
the result of white dwarf
- shortly after peak brightness as a magnitude (-3 star)
What is the variation in luminosity of nova stars
10^6
when a nova happens, the star become so bright that it can be seen without a telescope
What does Nova mean
new
What happens during a Nova
Occasionally, a star in the sky suddenly become between ten thousand and a million times brighter
- A hydrogen later compresses by the white dwarf gravitational forces (about 200 thousand times the gravitational force of earths surface). This enormous compression leads to the hydrogen fusion explosion . The explosion is the nova. In some binary systems the accretion explosion processes repeat periodically.
How frequent do Novas happen in our galaxy
about 20 times a year
2 or three of them been observed
What is an accretion ring
the hydrogen surface layer accumulated on the white dwarf , full of hydrogen
- this forms a companion star
What is hydrogen fusion
when hydrogen is converted into helium
- this is an exothermic reaction cuz the mass of hydrogen is greater than the mass of helium
What temperature does hydrogen need to get to be turn into helium? what happens after
100 million degrees
- the helium can be made which creates carbon
- this fusion helps the star to survive after main sequence
What is helium fusion
helium- carbon
What is carbon fusion
carbon- neon and magnesium
What is neon fusion and oxygen fusion
these reactions produce the elements of sulphur, silicon, phosphorus, up to iron
Which form of fusion is negative
only iron fusion
What are intermediate -mass stars and high mass stars
Mass= 8 solar masses and 25 solar masses (high)
What is the main sequence lifetime for intermediate and high mass stars
less massive = 500 million years or less
massive = few millions of years
What thermonuclear reactions happen after the main sequence of intermediate and high mass stars
complete set of reactions
- form hydrogen fusion to silicon to iron
- iron does not fuse cuz requires more energy than what is produced
What is the terminal state of intermediate and high mass stars
- supernova explosion
- can be seen from one end of the galaxy to the other
- the luminosity is greater than the combined luminosity of the entire galaxy for a short period of time
What happens after supernova explosion for intermediate and high mass stars
intermediate- neutron star
high mass- black hole
What leads to luminous supegiants
a serious of fusion reactions in high mass stars
L=10^5L
- can be as big as jupiters orbit
What is a supernova explosion
the death of a supermassive star
- the very last moments of a dying massive star
roughly 3000km in size
- the iron core collapses in about 1/10th second, the temp exceeds 5 billion L and the iron is converted back into protons, neutrons and electrons
What happens to the nuclear density during supernova explosion
increases by alot
What happens to the core during supernova explosion
becomes one huge stuff nucleus and the collapse stops
What stage comes after supernova explosion
core bounce
What factor does luminosity increase by inn supernova explosions
by a factor of up to 10^8
How much energy is emitted by a supernova
as much as all 200 billion stars in the milky way combined
- for a few days the supernova shines as brightly as an entire galaxy
What is a neutron star
- roughly 20 km diameter
- small
- 2 solar masses
- no thermonuclear reactions
What is escape velocity
one half of the speed of light
Who discovered pulsars and what are they
Jocelyn Bell and her professor anthony Hewish
- intense pulsating sources of radio energy known as pulsars
What are pulsars an example of
examples of neutron stars
Are neutron stars strong
yes very
What are neutron stars surrounded by
Regions of charged particles
- this magnetic field makes particles accelerate, they create radiation (radio waves)
What is the Crab Nebulal
the crab nebula and a pulsar are remains of 1054 AD supernova
- this is from a supernova explosion
- can be detected by radio, visible, UV, x rays and gamma rays
What is beamed radiation?
this is a path of light beam that extends from the poles of a neutron star
- this is only visible in the beam of light is directly pointed at the earth
Are neutron stars always visible
no because the beam of light is usually not facing the earth
is a white dwarf a star
technically no
