AST1002 Final Exam Review UF Question and Answers

What is the approximate rotation period of the sun? - ANSWER 25 days (equator) 36 days (poles) What is the approximate mean density of the Sun? - ANSWER 1410 kg/m3 What is the approximate surface temperature of the Sun? - ANSWER 5780 K How do we determine the luminosity of the Sun? - ANSWER L=Flux* 4 π R² What is the temperature of the core of the Sun? - ANSWER 15 million K What mechanism produces the energy in the core of the Sun? - ANSWER 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? - ANSWER 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? - ANSWER The photosphere of the sun forms the continuous spectrum How can we see the solar corona? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER "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? - ANSWER "transverse" motion is perpendicular to the line of sight What are the two axes of the HR diagram? - ANSWER Luminosity and temperature Where are the low and high temperature stars located in the HR diagram? - ANSWER 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? - ANSWER small radius stars are located on the bottom left and large radius stars are located on the top right. What is the spectroscopic parallax? - ANSWER 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: - ANSWER 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? - ANSWER Lstar= (Rstar/Rsun)^2 * (Tstar/Tsun)^4 * Lsun What parameters does mass of a star determine? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER Helium begins to fuse into Carbon at >108 K (100 million K). Temperature necessary to fuse Carbon? - ANSWER It will need to reach a temperature of 600 million K to ignite carbon How long will the Sun stay in the main sequence? - ANSWER 10 billion years At what point in the star evolution it will move off the main sequence? - ANSWER 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? - ANSWER sub-giant branch When the Sun moves off the main sequence, does surface temperature increases or decreases? What happens to the diameter ? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER White dwarf and planetary nebula Final object of a star with more than 1.4 solar masses but less than 3 solar masses? - ANSWER neutron star Final object for a star with more than 3 solar masses? - ANSWER BLACK HOLE Size of a white dwarf? Size of a neutron star? Size of a black hole? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER Cepheid variables and derived the distance local group of galaxies - ANSWER 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? - ANSWER 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? - ANSWER No, due to the dark matter problem What is the evidence of dark matter in the outer part (beyond 15 kpc) of the MW? - ANSWER It doesn't follow the Keplerian curve •Do we know what dark matter is? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 4.1 million solar masses Which type of galaxies are more massive? - ANSWER Giant ellipticals are bigger in size and more massive than spirals. Dust and gas in an elliptical galaxy? - ANSWER Ellipticals contain little or no gas and dust Dust and gas in a spiral galaxy? - ANSWER Yes Where can we find formation of new stars? In an elliptical galaxy? In a spiral galaxy? - ANSWER Usually in spirals in the spiral arms. Is the "tuning fork" an evolutionary sequence in galaxies? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER To estimate the mass use Newton's version of Kepler 3rd Law How can we estimate the age of the Universe? - ANSWER 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? - ANSWER No How are the wavelengths of light emitted by galaxies affected by the expansion of space? - ANSWER 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? - ANSWER 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? - ANSWER 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? - ANSWER The radiation has been redshifted by the expansion of the Universe What is Dark energy? - ANSWER 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? - ANSWER ? What are the names of the two points in the celestial sphere where the Sun crosses the celestial equator? - ANSWER Autumnal and Vernal Equinox Declination of the Sun for summer and winter solstice? - ANSWER +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? - ANSWER 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? - ANSWER Solar: New moon Lunar: full moon Why we don't see a solar or lunar eclipse every month? - ANSWER 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? - ANSWER The moon's orbit around Earth is an ellipse. What causes the seasons? - ANSWER The seasons are caused by Earth's 23.5 degrees tilted axis. Is it the changing distance of the Earth to the Sun? - ANSWER 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? - ANSWER 23.5 degrees