ASTRO (Science) EXAM 4 FINAL EXAM
QUESTIONS CORRECTLY ANSWERED.
How can we see the center of the galaxy? What is adaptive optics and why do
we need adaptive optics (AO) to see the center of the galaxy? Which
wavelengths do we use to measure the properties of the galactic center?
The Galactic Center lies 28,000 light-years away from the Sun, hidden by layers of
dusty spiral arms. Visible light cannot penetrate. Use infrared, radio, and X-rays to
image it. Stars at galactic center: imaged using adaptive optics. Map their motions
over the years, use Newton's laws to figure out the mass at the center. Air
turbulence in the atmosphere distorts light. That is why the stars appear to "twinkle".
Angular resolution is degraded. It is possible to "de-twinkle" a star. The wavefronts
of a star's light rays are deformed by the atmosphere. By monitoring the distortions
of the light from a nearby bright star (or a laser): a computer can deform the
secondary mirror in the opposite way and the wavefronts, when reflected, are
restored to their original state. This improvement is critical for measuring the
positions of distinct stars in the tiny region at the Galactic center. We use X-ray,
radio, and infrared wavelengths to study the properties of the galactic center.
How do we know that the universe is expanding? What is the relationship
between redshift and the expansion of the universe? How do we measure the
redshift of a distant galaxy?
We know the universe is expanding through motions of galaxies (redshifts) and
distances of galaxies (distance indicators/standard candles). We measure the
spectra of galaxies and the spectra of external galaxies are systematically
redshifted. Radiation from young, massive stars ionizes Hydrogen, gas is heated to
10,000 K, emission from H, O (other metals) is highlighted. We then measure the
redshift using the formula.
How do we measure the distance to far-away galaxies? What is a standard
candle?
We can measure the distances to far away galaxies by using Cepheids, the Tully
Fisher Relation, and Type Ia Supernovae. A standard candle is an object with known
constant luminosity.
,What is the best method for measure close distances? What is main-sequence
fitting? What are Cepheid variables? How can they be used to measure
distances? How can we measure the distances to Type Ia supernovae?
The best methods for measuring close distances are parallax and main-sequence
fitting. For a cluster of stars, we can compare the entire main sequence. We know
distance to Hyades cluster from parallax. By measuring how much fainter Pleiades
cluster MS is, we can calculate its distance. This is called main-sequence fitting.
Cepheids are pulsating stars with pulsation period ranges from one to a few
hundred days. Period of Cepheid closely related to luminosity. Cepheids: measure
period which tells you luminosity which can measure flux and you can then compare
flux and luminosity to infer distance to galaxy containing Cepheids. Since every
Type Ia SN progenitor is likely the same size (~1.4 solar
masses), they may be good standard candles for measuring distance.
What is Hubble's Law, and what is the Hubble Diagram? What is the Hubble
constant? What is cosmological redshift?
It was known that galaxy spectra tended to show redshifts. • Hubble found that a
galaxy's recession velocity (v=cz) was proportional to the distance (d) away from
us! More distant galaxies moving away more quickly. Ho, the slope of the line, is
called Hubble's constant [km/s/Mpc]. Hubble's law required measurements of both
redshifts and distances. Is a natural consequence of an expanding universe. H0 =
72 km/s/Mpc
Cosmological redshift is redshift is due to the expansion of the universe, which
stretches the wavelength of light passing between galaxies. Light from more distant
objects travels for a greater time (because light has a finite speed), the Universe
expands more while the light travels to us, and the wavelengths get more stretched.
How can we relate the redshift of an object and the size of the universe when
that light was emitted? How about the density of the universe then?
The important formula is:
observed wavelength/emitted wavelength=
(size of the Universe now)/(size when the light was emitted). Now we're saying that
the redshift tells you by how much the Universe has expanded in the time it's taken
for the light to get to us.
, Has Hubble's constant stayed the same throughout history? What does the
Hubble constant say about the expansion of the universe throughout time?
Hubble's Constat has changed throughout history. Use redshifts and distances to
probe how size of universe and H(t) evolves with time, i.e. redshift vs. distance can
be re-cast as expansion factor (size of Universe) vs. lookback time.
How can we interpret the expansion of the universe with kinetic and potential
energies? What does it mean to say that the universes expansion is bound or
unbound?
To understand dynamics of expansion of universe, think of projectile. It has kinetic
energy (KE) and gravitational potential energy (PE), and the sum of KE+PE remains
constant. Kinetic energy of sphere is related to Hubble expansion. Mass inside the
sphere is M=density×volume=ρ×V. Gravitational potential energy is related to
density (ρ) inside sphere. We refer to the mass density required for this
gravitational pull to equal the kinetic energy of the Universe as the critical density.
• if density > critical density, Universe will stop expanding and then contract
(bound)
• if density < critical density, Universe will expand forever (unbound)
• if density = critical density, Universe will stop expanding at time=infinity (critical)
What is the critical density of the universe, and what does this density mean for
the eventual fate of the universe?
The value of H0 tells us about the current kinetic energy of the universe. Based on
what we know about the current rate of expansion in the universe, the critical
density is ρcrit~10-26 kg/m3 (several atoms per cubic meter, on average). It
determines the number based off whether the universe will be bound, unbound, or
equal it. Space time can only be flat when the average density of matter is close to
the critical density.
How does matter dictate the shape of the universe? What are the possible curva-
tures of space-time, and how do they relate to the matter density in the
universe?
Matter can shape the curvature of space time. According to the theory of general
relativity, the overall geometry depends on the average density of matter and
energy in the universe. The possible curvatures are:
Flat- Combined density of matter plus energy is precisely equal to a value known
QUESTIONS CORRECTLY ANSWERED.
How can we see the center of the galaxy? What is adaptive optics and why do
we need adaptive optics (AO) to see the center of the galaxy? Which
wavelengths do we use to measure the properties of the galactic center?
The Galactic Center lies 28,000 light-years away from the Sun, hidden by layers of
dusty spiral arms. Visible light cannot penetrate. Use infrared, radio, and X-rays to
image it. Stars at galactic center: imaged using adaptive optics. Map their motions
over the years, use Newton's laws to figure out the mass at the center. Air
turbulence in the atmosphere distorts light. That is why the stars appear to "twinkle".
Angular resolution is degraded. It is possible to "de-twinkle" a star. The wavefronts
of a star's light rays are deformed by the atmosphere. By monitoring the distortions
of the light from a nearby bright star (or a laser): a computer can deform the
secondary mirror in the opposite way and the wavefronts, when reflected, are
restored to their original state. This improvement is critical for measuring the
positions of distinct stars in the tiny region at the Galactic center. We use X-ray,
radio, and infrared wavelengths to study the properties of the galactic center.
How do we know that the universe is expanding? What is the relationship
between redshift and the expansion of the universe? How do we measure the
redshift of a distant galaxy?
We know the universe is expanding through motions of galaxies (redshifts) and
distances of galaxies (distance indicators/standard candles). We measure the
spectra of galaxies and the spectra of external galaxies are systematically
redshifted. Radiation from young, massive stars ionizes Hydrogen, gas is heated to
10,000 K, emission from H, O (other metals) is highlighted. We then measure the
redshift using the formula.
How do we measure the distance to far-away galaxies? What is a standard
candle?
We can measure the distances to far away galaxies by using Cepheids, the Tully
Fisher Relation, and Type Ia Supernovae. A standard candle is an object with known
constant luminosity.
,What is the best method for measure close distances? What is main-sequence
fitting? What are Cepheid variables? How can they be used to measure
distances? How can we measure the distances to Type Ia supernovae?
The best methods for measuring close distances are parallax and main-sequence
fitting. For a cluster of stars, we can compare the entire main sequence. We know
distance to Hyades cluster from parallax. By measuring how much fainter Pleiades
cluster MS is, we can calculate its distance. This is called main-sequence fitting.
Cepheids are pulsating stars with pulsation period ranges from one to a few
hundred days. Period of Cepheid closely related to luminosity. Cepheids: measure
period which tells you luminosity which can measure flux and you can then compare
flux and luminosity to infer distance to galaxy containing Cepheids. Since every
Type Ia SN progenitor is likely the same size (~1.4 solar
masses), they may be good standard candles for measuring distance.
What is Hubble's Law, and what is the Hubble Diagram? What is the Hubble
constant? What is cosmological redshift?
It was known that galaxy spectra tended to show redshifts. • Hubble found that a
galaxy's recession velocity (v=cz) was proportional to the distance (d) away from
us! More distant galaxies moving away more quickly. Ho, the slope of the line, is
called Hubble's constant [km/s/Mpc]. Hubble's law required measurements of both
redshifts and distances. Is a natural consequence of an expanding universe. H0 =
72 km/s/Mpc
Cosmological redshift is redshift is due to the expansion of the universe, which
stretches the wavelength of light passing between galaxies. Light from more distant
objects travels for a greater time (because light has a finite speed), the Universe
expands more while the light travels to us, and the wavelengths get more stretched.
How can we relate the redshift of an object and the size of the universe when
that light was emitted? How about the density of the universe then?
The important formula is:
observed wavelength/emitted wavelength=
(size of the Universe now)/(size when the light was emitted). Now we're saying that
the redshift tells you by how much the Universe has expanded in the time it's taken
for the light to get to us.
, Has Hubble's constant stayed the same throughout history? What does the
Hubble constant say about the expansion of the universe throughout time?
Hubble's Constat has changed throughout history. Use redshifts and distances to
probe how size of universe and H(t) evolves with time, i.e. redshift vs. distance can
be re-cast as expansion factor (size of Universe) vs. lookback time.
How can we interpret the expansion of the universe with kinetic and potential
energies? What does it mean to say that the universes expansion is bound or
unbound?
To understand dynamics of expansion of universe, think of projectile. It has kinetic
energy (KE) and gravitational potential energy (PE), and the sum of KE+PE remains
constant. Kinetic energy of sphere is related to Hubble expansion. Mass inside the
sphere is M=density×volume=ρ×V. Gravitational potential energy is related to
density (ρ) inside sphere. We refer to the mass density required for this
gravitational pull to equal the kinetic energy of the Universe as the critical density.
• if density > critical density, Universe will stop expanding and then contract
(bound)
• if density < critical density, Universe will expand forever (unbound)
• if density = critical density, Universe will stop expanding at time=infinity (critical)
What is the critical density of the universe, and what does this density mean for
the eventual fate of the universe?
The value of H0 tells us about the current kinetic energy of the universe. Based on
what we know about the current rate of expansion in the universe, the critical
density is ρcrit~10-26 kg/m3 (several atoms per cubic meter, on average). It
determines the number based off whether the universe will be bound, unbound, or
equal it. Space time can only be flat when the average density of matter is close to
the critical density.
How does matter dictate the shape of the universe? What are the possible curva-
tures of space-time, and how do they relate to the matter density in the
universe?
Matter can shape the curvature of space time. According to the theory of general
relativity, the overall geometry depends on the average density of matter and
energy in the universe. The possible curvatures are:
Flat- Combined density of matter plus energy is precisely equal to a value known
