AST1002 Final Exam Review UF

What is the approximate rotation period of the sun?

25 days (equator)

36 days (poles)

What is the approximate mean density of the Sun?

1410 kg/m3

What is the approximate surface temperature of the Sun?

5780 K

How do we determine the luminosity of the Sun?

L=Flux* 4 π R²

What is the temperature of the core of the Sun?

15 million K

What mechanism produces the energy in the core of the Sun?

In the core, energy is produced by hydrogen atoms being converted into molecules of helium.

What is the minimum temperature needed to start fusing H into He?

The temperature of the core must be least 10 million K in order to be able to convert H into

He.

What type of spectrum does the Sun emit?

The photosphere of the sun forms the continuous spectrum

How can we see the solar corona?

The solar coronal plasma has enough temperature (kinetic energy) to escape the Sun's gravity

How long does it take for the electromagnetic radiation to reach the Earth?

The radiation emitted by the Sun travel at the speed of light and take about 8 minutes to reach Earth.

How long does it take for the solar wind and particles ejected from the Sun to reach the Earth?

The radiation emitted by the Sun travel at the speed of light and take about 8 minutes to reach Earth.

What do we need to know to determine the distance to a star using the stellar parallax?

Knowing the value of the AU in km, we use the stellar parallax, to find distances to "nearby" stars

How can we determine the radial motion of a star?

"line of sight" or radial motion is measured through Doppler shift of emission/absorption lines

How can we determine the transverse motion of a star?

"transverse" motion is perpendicular to the line of sight

What are the two axes of the HR diagram?

Luminosity and temperature

Where are the low and high temperature stars located in the HR diagram?

Low-temperature stars are located to the right and high-temperature stars are located to the left.

Where is a large radius and small radius star located in the HR diagram?

small radius stars are located on the bottom left and large radius stars are located on the top right.

What is the spectroscopic parallax?

The spectral analysis provides information to determine the temperature of the star or the spectral classification. If we know the

temperature for a main sequence star then we can deduce the luminosity

Steps to determine the distance d using spectroscopic parallax:

Measure the flux, determine the surface

temperature (or spectral classification), use the HR diagram to determine the luminosity, use the equation: Flux = Luminosity/ 4^d 2

How can you determine the luminosity L of a star (in solar units) respect to the Sun?

Lstar= (Rstar/Rsun)^2 (Tstar/Tsun)^4 Lsun

What parameters does mass of a star determine?

Luminosity, radius, surface temperature, lifetime, evolutionary phases.

A star of smaller mass lives a shorter or a longer life respect to a large mass star?

A star with a smaller mass lives a longer lifetime.

How long will take for the Sun to convert all the H into He until all the H is used up?

In the Sun's core, the conversion of

H to He will take about 10 billion years.

What is the temperature necessary to fuse He?

Helium begins to fuse into Carbon at >108 K (100 million K).

Temperature necessary to fuse Carbon?

It will need to

reach a temperature of 600 million K to ignite

carbon

How long will the Sun stay in the main sequence?

10 billion years

At what point in the star evolution it will move off the main sequence?

When the core hydrogen is used up and it begins to fuse helium.

To what region in the HR diagram will be Sun move once it has used all the H?

sub-giant branch

When the Sun moves off the main sequence, does surface temperature increases or decreases? What

happens to the diameter ?

When a star moves off the main sequence, the hot shell causes the outer layers to expand and cool.

Why can the sun not fuse carbon?

Solar mass stars cannot squeeze and heat the

core enough to ignite Carbon.

What happens to the Sun once it use all the H and He and ends up with a core of carbon?

With no more production of energy in

the core, the carbon core continues to

contract and heat and collapses into a white dwarf.

What is a planetary nebula?

The expanding emission line nebula heated by intense radiation from the hot white dwarf

What is a white dwarf? What is the chemical element that compose a white dwarf?

A white dwarf is a small very dense star that is typically the size of a planet. It is composed of composed of carbon and oxygen and the outside is hydrogen and helium.

Why can a star more massive than the Sun fuse heavier elements such as carbon, oxygen, neon?

Because gravity squeezes and heats the

core. The temperature increases enough to be able to ignite Carbon and then other elements.

What happens when a massive star try to fuse iron?

Creating elements heavier than Iron requires energy, so the star collapses and becomes a neutron star.

What is the final object resulting from the collapse of a star of one solar mass?

White dwarf and planetary nebula

Final object of a star with more than 1.4 solar masses but less than 3 solar masses?

neutron star

Final object for a star with more than 3

solar masses?

BLACK HOLE

Size of a white dwarf? Size of a neutron star? Size of a black hole?

A white dwarf is about the size of Earth. A neutron star is not much bigger than a small city. Stellar-mass black holes are typically in the range of 10 to 100 solar masses, while the supermassive black holes at the centers of galaxies can be millions or billions of solar masses.

What basic principle can be applied to understand why a neutron star rotates fast and why it has an intense magnetic field?

When the star collapses into a neutron star, the plasma (electrically charged ) that make up the core of the star carries and intensify the magnetic field

What is a pulsar?

The central object (core of star) is a rotating

neutron star with a strong magnetic field.

What would happens to the Earth's orbit if the Sun could collapse into a black hole?

Nothing because the Earth will be at the same distance and it will feel the same gravitational attraction from the Sun at 1 AU.

Why can light emitted at the event horizon or inside the event horizon not escape a black hole?

The escape velocity becomes equal to the speed of light. The trajectory of a light beam will be so distorted. Nothing can get out including light.

How it is possible to estimate the mass of the supermassive black hole in the center of the Milky Way?

Newton version of Kepler 3rd

Law! MBH = a^3/P^2

What is the relationship between the two parameters that makes the Cepheids and RR Lyrae useful to measure distances?

The relationship between

the pulsation period and its luminosity. We plot the pulsation period and then we measure these periods and find the luminosity. Using the luminosity we find the distance.

How was Hubble able to determine the distance to the Andromeda galaxy?

Cepheid variables and derived the distance

local group of galaxies

Hubble found that galaxies outside the Local Group are all moving away from us

What should happen to the rotational curve of the MW beyond the visible disk if no dark matter was

present?

The rotational curve reveals the presence of

"invisible" matter . That unseen matter is only detected by its gravitational effect on stars or object inside the orbit of those objects.

Does the velocity follow a Keplerian curve beyond the 15 kpc distance?

No, due to the dark matter problem

What is the evidence of dark matter in the outer part (beyond 15 kpc) of the MW?

It doesn't follow the Keplerian curve

•Do we know what dark matter is?

Dark matter cannot be detected by the light it emits and is only detected by gravitational attraction but there is a lot of dark mass in the outer part of the galaxy (about 2/3)

What are the possible candidates to explain the existence of dark matter?

White dwarf stars, very low mass stars (red dwarfs), brown dwarfs, neutron stars and black holes, Massive Compact Halo Objects (MACHOs), Exotic sub-atomic particles- Weakly Interacting Massive Particles (WIMPs)

How do we know that there is a black hole in the center of our galaxy?

Because the stars of our galaxy are revolving around an object that is not emitting light

What method can be used to determine the mass of the black hole in the center of the MW?

Using Kepler's third law we can find the mass using the orbital period of a star around the black hole and how far the star is from the black hole.

How many solar masses is estimated to have the supermassive black hole in the center of the MW?

4.1 million solar masses

Which type of galaxies are more massive?

Giant ellipticals are bigger in size and more massive than spirals.

Dust and gas in an elliptical galaxy?

Ellipticals contain little or no gas and dust

Dust and gas in a spiral galaxy?

Yes

Where can we find formation of new stars? In an elliptical galaxy? In a spiral galaxy?

Usually in spirals in the spiral arms.

Is the "tuning fork" an evolutionary sequence in galaxies?

No, galaxies do not appear to evolve from one galaxy to another.

How can we use the Hubble law to determine distances to far away galaxies?

Redshift and Hubble's Law - measuring distances to objects far, far away. V = Ho D

What do we need to measure in those galaxies to determine their distance and apply Hubble law?

The velocities, which can be measured from the cosmological red shift of the emission/absorption lines

How can we explain the large luminosity of a quasar and what was the unknown emission lines in the spectrum of a quasar?

The unfamiliar lines of a quasar were normal Hydrogen lines being seen at much higher redshifts than any of Hubble's galaxies, thus luminosity is due to the massive distance.

What method can be used to determine the mass of the black hole in a quasar?

To estimate the mass use Newton's version of Kepler 3rd Law

How can we estimate the age of the Universe?

The age is calculated by measuring the distances and radial velocities of other galaxies, most of which are flying away from our own at speeds proportional to their distances.

Does the Universe has a center?

No

How are the wavelengths of light emitted by galaxies affected by the expansion of space?

As the Universe expanded, the wavelength of

radiation from the Big Bang also expanded, i.e.

it became redshifted and thus cooled off.

On a small scale, such as the Local Group, are galaxies showing cosmic redshift? What

force is involved in the local group that keeps galaxies together?

Instead of expanding along with the rest of the universe because the gravitational attraction between the two is stronger. In fact, the entire Local Group is collapsing under the gravitational attraction, and in about 3 billion years, the Milky Way and Andromeda will collide.

What is the temperature of the CMB? In which part of the spectrum (wavelength) peaks

the CMB emission?

The spectrum of this background radiation is of a blackbody with a temperature of 2.7 K. Orginally gamma waves but now peaked in radio/microwaves

What happened to the radiation that

now is in the microwave range?

The radiation has been

redshifted by the

expansion of the

Universe

What is Dark energy?

In 1998, astronomers found that the universe may actually be accelerating. The repulsive effect that caused the acceleration is called "Dark Energy"

Why it has been necessary to postulate the presence of Dark Energy?

To explain the observed accelerated expansion of the universe, which cannot be accounted for by matter and gravity alone.

What are the names of the two points in the celestial sphere where the Sun crosses the celestial

equator?

Autumnal and Vernal Equinox

Declination of the Sun for summer and winter solstice?

+23.5, -23.5

Where is the Sun respect to the Moon when it is full Moon. Where is the Sun when it is new Moon?

Full Moon: blocked by earth

New Moon: Behind moon and earth

Where is the Moon respect to the Sun's position for a lunar eclipse? For a solar eclipse?

Solar: New moon

Lunar: full moon

Why we don't see a solar or lunar eclipse every month?

Eclipses occur only when the Moon crosses the ecliptic (5 degrees) while it is a new or full moon

Why are there annular solar eclipses?

The moon's orbit around Earth is an ellipse.

What causes the seasons?

The seasons are caused by Earth's 23.5 degrees tilted axis.

Is it the changing distance of the Earth to the Sun?

Earth is closer to the sun during winter in the north hemisphere and further from the sun during summer in the north hemisphere.

What is the value of the tilt angle of the rotational axis of the Earth respect to the perpendicular to

the orbital plane?

23.5 degrees

What is the Earth's precession? What causes the Earth's precession?

Earth's precession is that it is slowly spinning down like a top, this is due to gravity that causes Earth's axis to slowly precede

What is the effect of the precession in the position of the star Polaris?

It will change how we see the star in the sky

What is the duration of the precession cycle

26,000 years

What is stellar parallax?

Stellar parallax the apparent shift of position of any nearby star against the background of distant objects.

How can we use stellar parallax to calculate distance to a nearby star?

If we measure the angle of displacement of the object and we know the length of the baseline (Earth diameter) we can calculate the distance to the object. The amount of parallax is inversely proportional to an object's distance

How was retrograde motion explained in the geocentric model?

Using Epicycles (small circles around earth) and Deferents (large circles around earth)

How do we explain the retrograde motion of Mars under the heliocentric model?

The planet appeared to undergo retrograde motion when the Earth approaches and overtakes the planet into its orbit

What was Tyco Brahe's contribution to astronomy?

He built instruments to make detailed observations of space without using a telescope.

What are the 3 Kepler 's laws of planetary motion?

-1st Law (shapes): Planets orbit the Sun in elliptical orbits. (Perihelion: shortest distance from the sun & Aphelion: largest distance from the sun)

-2nd Law: The line connecting a planet with the Sun sweep equal areas in equal times. When close to the sun, it moves much faster

When farther from the sun, moves slower

-3rd Law: p² = a³. The square of a planet's orbital period is proportional to the cube of its semimajor axis. Modified 3rd law by Newton: p² = a³/M, M=(m1+m2)

What are the 3 Newton's law a motion?

If one of the masses is larger, the gravitational force is larger, and vice versa. The further the distance the less gravitational force on the object. A. If two objects pulled with same force, one with GREATER mass will accelerate LESS

B. If two identical objects are pulled with different forces, the one pulled with greater force will accelerate more

What are four important discoveries Galileo did with his telescope and how some of them supported the

heliocentric model

-The moon has mountains and craters

-The sun has imperfections such as sun spots, and based on the fact that these spots were in different positions by day means the sun rotates

-Discovered the 4 moons of Jupiter, and they orbit Jupiter just like our moon orbits Earth. This supports notion that Earth is NOT center of universe

-Venus shows a complete cycle of phases. Similar to monthly changes of our moon

Concept of mass and weight. How your mass change if you are on the Moon? How is your

weight change is you are on the Moon?

-Mass does not equal weight

-Mass does not change ever, weight does

Moon's force of gravity is 1/6th that of Earth. If you weigh 100 lbs. on Earth, you would weigh 16 lbs on the moon

How do we determine the value in km of the AU?

1 AU= distance from the Earth to sun = 150 million km= 93 million miles

We determine this using radar (bounced light off of venus and times how long it took to get back)

Why do astronomers build bigger telescopes? Any advantage of having bigger telescopes?

They are capable of greater resolution and seeing farther. Both the light-collecting area and the angular resolution are increased by a larger lens or mirror.

What is the objective of a reflecting telescope? Of a refracting telescope?

A reflecting telescope focuses light with mirrors which forms an image. A refracting telescope focuses light with lenses, the lends bends the light to create an image.

What is the focal length of a telescope?

The focal length of a telescope is the equivalent distance from the lens or mirror to the plane where the image forms.

What is the dependence of the diameter of a telescope with the light gathering power?

Telescopes with larger lenses or mirrors have a larger collecting power and will act as a light bucket.

What is the dependence of the diameter with the angular resolution of a telescope?

Telescopes with larger lenses or mirrors are able of taking photos with greater resolution and detail.

If the separation between two stars in a binary system is much smaller than the resolution of the telescope, will you be able to see those two star separated? Why?

No, it will look like a single image because of the diffraction of light (an airy disk).