Exam 3 to End Flashcards
Going up the main sequence (red to blue stars) mass, radius and luminosity all…
Increase
Which stars are more luminous: high-mass or low-mass?
High-mass stars
High-mass stars burn their fuel supply much _____ than low-mass stars.
Faster
Star lifetimes:
Red dwarf (0.1 M@) —->
Sun (1 M@) —->
Blue supergiant (10 M@) —->
Star lifetimes:
>10^12 yr
10^10 yr
2 x 10^7 yr
Which stars move through all phases faster: high-mass or low-mass?
High-mass stars
Where do stars form?
In cold, dense interstellar clouds
What is a dark nebula?
A dense cloud of gas and dust, which absorbs visible light and obscures stars behind it.
What is cloud collapse and what causes it?
When a cloud goes out of hydrostatic equilibrium. This is caused by gravity, meaning the cloud is compressed by a shockwave.
What are possible triggers of a cloud collapse?
A nearby supernova, a cloud passing through a galaxy’s spiral arm, or cloud collision
True of false: As gravitational contraction of a cloud continues, gas is compressed and heated.
True
Protostars are surrounded by what (which is also where they form)?
Dense gas and dust clouds
How are protostars best observed?
In infrared
Which is less absorbed by dust: Visible light or infrared?
Infrared
When does H-fusion begin in the core of a protostar?
When its central temperature reaches 10^7 K
Which stage begins a star’s main sequence?
A protostar
What does an H-R diagram show?
A star’s evolutionary track as it’s luminosity and temperature change.
What kind of luminosity and temperature does a protostar have?
High luminosity and low temperature
What is a reflection nebulae?
A bluish haze seen around hot, young stars due to interstellar dust.
A reflection nebulae’s dust reflects blue or red light more effectively?
Blue light (the red light appears redder than its true color)
What is interstellar reddening?
When dust clouds between a star and its observer scatter more blue light than red light out of its beam, which makes the star appear redder than its true color.
Why does Earth’s sky appear blue?
Because air molecules reflect more blue light than red light.
Why does the sun look red near the horizon?
Because more blue light than red light has been removed and the light travels through more atmosphere near the horizon.
What are lower end main sequence limits?
Below 0.08 M@ when the central temperature is too low for fusion
What are upper end main sequence limits?
Above about 100 M@ when the radiation pressure blows stars apart
What are the evolutionary stages of a 1 M@ Star?
- Main sequence
- Red giant
- Helium flash
- Horizontal branch
- Second red giant
- Planetary nebula
- White dwarf
Nuclear fusion in a main sequence star’s core converts…
Hydrogen to Helium
When does a star leave the main sequence?
When the hydrogen is used up in its core
How long will the sun spend on its main sequence?
10^10 years
True or false: More massive stars burn through Hydrogen slower?
False, they burn through it faster.
When H fusion ends in a star’s core, does it continue somewhere else during the red giant phase?
Yes, in the shell around the core.
What causes a star’s core to contract and heat during the red giant phase?
Gravity
During the red giant phase, the star’s outer region…
Expands and cools due to increased pressure
What are some red giant properties?
Big, bright, and cool
R = 100 R@ = 0.5 AU
L = 1000 L@
T = 3500 K
What is a helium flash?
When the central temperature of a red giant is 10^8 K and Helium fusion begins in its core.
What is the triple-a process?
3 He —-> C + energy
He = helium nucleus = a-particle C = carbon nucleus
What fuses in the horizontal branch phase of a star?
Helium fuses to carbon in its core, some Helium fuses with carbon to form oxygen.
What does the star’s core do in the horizontal branch phase?
It expands, cools and becomes bluer.
Which luminosity is higher: the sun or a horizontal branch star?
A horizontal branch star’s luminosity
True or false: Horizontal branch stars form a horizontal grouping in the H-R diagram.
True
What stage is a star in when there is no Helium left in the core and its core is now made up of carbon and oxygen?
Second red giant phase
In the second red giant phase, what does the core do and what does the envelope do?
The core contracts and heats, while the envelope expands and cools.
Towards the end of the second red giant phase, the star becomes…
a red supergiant
During the planetary nebula phase, what does the red supergiant’s envelope do?
It pulsates unstably, eventually becoming ejected and exposing the star’s core.
What is the structure of the star during its planetary nebula phase?
It has an expanding spherical gas shell around the small, hot star and the shell appears as a bright ring.
The structure of other planetary nebulae’s ejected gas can be nonspherical due to…
A star orbiting a binary companion, or a disk around the star producing an hourglass shape.
Hot, dense core becomes a white dwarf =
collapsed star about the size of the Earth
What is the density of a white dwarf?
10^9 kg/m^3 = 10^6 x density of water
What is the structure of a white dwarf?
Electrons are packed as closely as possible, star does not contract further, and temperature and luminosity decrease in time.
What is the Chandra limit?
The maximum possible mass for a white dwarf = 1.4 M@
Which kind of star must collapse to a smaller size and higher density?
A more massive burned-out star
Which objects are highly collapsed?
Neutron stars and black holes
What is an open star cluster?
a cluster of young, recently formed stars found in the disk of a Galaxy where most stars are near the main sequence.
What is the age of an open star cluster?
2 x 10^7 years
What is a globular star cluster?
a cluster of only old stars (greater than 10^10 yr) found in the bulge/halo of our Galaxy, with almost no stars present on the upper main sequence (M>M@).
What is a main sequence turnoff?
the location in the H-R diagram of stars just becoming red giants. The younger the cluster, the higher the mass of the turnoff stars.
What is an equinox?
An event that occurs twice a year where the day and night have equal length. The sun is overhead at equator on equinoxes.
When do equinoxes occur?
The autumnal (fall) equinox occurs around Sept. 22nd and the vernal (spring) equinox occurs around March 21st.
When is the day longer and when is the day shorter during the year?
The day is longer in the summer and shorter in the winter.
In which direction does the sun rise and set?
The sun rises due East and sets due West.
What is a solstice?
An event that occurs twice a year when the Sun reaches its highest or lowest excursion relative to the equator.
When do solstices occur?
Summer solstice (LONGEST DAY OF THE YEAR) occurs around June 21st and winter solstice (SHORTEST DAY OF THE YEAR) occurs around Dec. 21st.
Where does the Earth’s axis point during summer solstice?
It points most TOWARD the Sun.
Where does the Earth’s axis point during winter solstice?
It points most AWAY from the Sun.
During the evolution of a high mass star (M>10M@), nuclear fusion produces elements up to…
Iron (but the core does not undergo fusion, it occurs in the shells surrounding the core)
In a high mass star (M>10M@), the core is the size of…
the Earth
When the core mass of a high mass star exceeds 1.4M@ (Chandra limit), it becomes…
Unstable
What is a supernova and how does it occur?
The death of a massive star. It occurs when gravity overcomes outward electron pressure in iron core, causing rapid collapse.
Electrons and protons combine in a supernova and form…
Neutrons (in nucleus of an atom), and then a pulse of neutrinos (very low mass particles) are produced.
What stops the core’s collapse in a supernova and when?
Neutron pressure, when the core is 20 km (12 mi) across. The pressure balances gravity again in the core.
Describe the supernova’s blast.
The core bounces outward and energy is released by its collapse, which causes a shockwave and the rest of the exploded star to travel out.
What kind of elements are produced in a supernova blast?
Elements heavier than iron are produced in a supernova blast.
What kind of elements are produced in massive stars?
Elements heavier than Hydrogen and Helium.
What do supernova blasts return Hydrogen and Helium to?
Interstellar gas
What was Supernova 1987A?
The closest supernova observed in the past 400 years.
Where was Supernova 1987A observed, when, and how far away was it?
It was observed in Large Magellanic Cloud (LMC) in Feb. 1987 and it was 160,000 ly away.
Describe the Supernova 1987A.
It was a blue supergiant that exploded. Its luminosity increased by a few 1,000x and had three bright rings which provided evidence for the hourglass wind flow of a doomed star.
What did the Chandra X-ray observatory find out about Cas A supernova?
That its remnant was an expanding cloud of hot gas, seen about 300 years after the supernova, about 10 ly in radius.
What is a type of stellar remnant from a supernova blast and describe its structure?
Neutron star, ultra-collapsed core, Mass is equal or greater than 1.4M@, Radius is 10 km or 6 mi, It is made of tightly packed neutrons.
True of false: Neutron stars can emit all forms of electromagnetic radiation: from radio to gamma-rays.
True
Describe the pulsar quality of neutron stars produced by supernova blasts.
The neutron star has a pulsing source of radio, visible, or x-ray. It was discovered by Bell in 1967. The pulse periods were 0.0014 sec to 10’s of sec., up to 700 flashes per sec. The first explanation was alien signals.
