Week 1-2: Overview of the Universe & the View from the Earth

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1 Week 1-2: Overview of the Universe & the View from the Earth Hassen M. Yesuf September 29, Lecture summary Protein molecules, the building blocks of a living organism, are made of C, N and O elements which are produced in stars by a nuclear fusion whereby the intense heat in the interior of the stars help overcome the electrostatic forces, between atomic nuclei, and join together (fuse) the light nuclei to a heavier nucleus. Even though the temperature of the universe was high at the big bang, due to the expansion of universe and its rapid cooling, only light elements such as H, He and Li were produced in the big bang. So now you know that a star is a big ball of gas that generates heat and light by a nuclear fusion. Stars are atleast about 15 times more massive than our planet Jupiter. The smallest stars are called brown dwarfs. On the hand, a planet (may be gaseous, rocky or icy) is less massive to have a nuclear fusion in its core. A planet, by definition, orbits its parent star and has cleared small objects in its way. A galaxy is a large collection of star, gas, dust and dark matter bound by their mutual gravity. A typical galaxy like our own Milky Way contains about few hundred billion stars. Galaxies come in many shapes and sizes, and are morphologically classified in three broad classes: spiral, elliptical and irregular. Our Milky Way galaxy and its neighbor Andromeda galaxy are just two of the billions of galaxies in our observable universe. According to the currently favored cosmological model, galaxies comes from small density variations in the early universe. These quantum variation were amplified by inflation and grew in size over time by gravity to be galaxies. The cosmic microwave background (CMB), the remnant light from Big Bang, detected first by COBE satellite and then by WMAP, is an evidence for the density fluctuations which galaxies seed from. Due to this bottom-up build up of the universe, galaxies tend to cluster into groups and interact or merge with each other. For instance, the Milk Way and Andromeda galaxy are two of about 40 galaxies in the Local Group which in turn is a small part of the Local Super-cluster, which itself is one small part of the universe. Stars are born when giant molecular clouds (GMC) of Hydrogen, in a galaxy, gravitationally collapse and fragment into clumps. When the interiors of these contracting clumps reach certain temperature and luminosity stars are born. Then, they successively fuse light elements to heavy elements (i.e change their composition and structure and evolve to different types of stars) as their temperature, pressure and density varies with time as a result of nuclear fusion (and gravity). At end of their life time, stars explode as supernovae and empty their enriched elements into the interstellar medium (the gas between stars in the galaxy) and the galaxy then recycle the elements into new stars and planets. The earth spins on its axis once per day. As a result, our Sun and other stars appear to rise and set once per rotation. Note that circumpolar stars do not set as they are always above horizon and cannot disappear from our view as the earth rotates. The axis of rotation of the earth changes with time, over many centuries, as the earth precesses like a top. Currently, the axis of rotation is tilted by 23.5 degree and this is responsible for our four seasons. 1

2 The celestial sphere is an imaginary sphere of arbitrarily large radius, concentric with the Earth and rotating upon the same axis. All objects in the sky can be thought of as projections upon the celestial sphere. Projected upward from Earth s equator and poles are the celestial equator and the celestial poles (Source: Wikipedia!). In this sphere, Declination is analogous to a latitude while Right Ascension is to a longitude. The Earth also revolves around the Sun over the period of one year. The constellations we observe vary with time of the year because our night sky is changing as we orbit the Sun. The phase of the Moon is changing because, as it orbit the Earth, its relative position to the Sun and the observer is changing. One half of the Moon facing the Sun is always illuminated and its this half that an observer can see fully or partially. For instance, the phase of the Moon is full when the observer can fully see the illuminated half moon (hemisphere) projected as a circle on the sky. You are required to understand how each phase is formed and when it rises and sets. Similarly, you need to understand how eclipses are formed. Eclipses are a formed when the Earth or the Moon blocks the sunlight that an observer see directly or reflected of the Moon. 2 Facts and figures Age of the universe: 14 billion years and age of the solar system: 4.5 billion years. 1 light year (ly) : 300, 000 km/s π s km so light year is not a measure of interval of time. Note that 1 yr π 10 7 s 1 AU : the distance from the Earth to the Sun which approximately 8 light minute or 150 million kilometers The solar system is about 40 AU with for instance Mars: 1.5 AU, Jupiter: 5 AU, Saturn 10 AU Earth : mean radius: 6400 km, average orbital speed: 107,200 km/hr, rotational speed: 1, 700 km/hr. Earth rotates on its axis once each day and orbits the Sun once each year and the earths axis is tilted by 23.5 degree. Moon: distance to the Moon 400, 000 km (1.3 light second) or AU, size of the Moon: about 0.3 of the radius of the earth Sun: made of Hydrogen and Helium and has orbital period of around 230 million years around the center of the Galaxy. The Sun is located about half way from the center of Milky Way (28,000 ly). Milky Way: has about 200 billion stars and most of the mass of the Milk Way is not visible (i.e most of the mass is not from stars in the disk but from the dark matter in halo as inferred from rotation velocity of the stars at different distance from the center of the Milk Way). 3 Comprehension/Review questions 1. What does our solar system consists of? Ans: the Sun and all the objects that orbit it (planets and asteroids but not stars and the Sun is the only star in our solar system) and the nearest star is about 4 ly away. 2. What do astronomers mean by the Big Bang? Ans: The event that marked the beginning of the expansion of the universe. Note that it does not mean the explosion of massive stars at the end of their life cycle or birth of solar system. The expansion of the universe 2

3 means that the average distance between galaxies is increasing with time and more distant galaxies move faster. Note also that the average distance between stars within a given a galaxy remains unchanged by the expansion. In other words, the stars in the galaxy are still bound by their mutual gravity when the universe expands. 3. Can we see a galaxy that is 20 billion light years away? Ans: No, because we live in a universe which is younger than 20 billion years and there is no galaxy that far away in our universe. 4. On a scale in which the distance from Earth to the Sun is about 15 meters, what is the distance from Earth to the Moon? Ans: small enough to fit within your hand. More precisely if you want: if 1 AU is to 15 m, what is AU to? 15 m AU/ 1 AU= m or 3.75 cm or 1.5 inch 5. How are galaxies important to our existence? Ans: Galaxies recycle material from one generation of stars to the next, and without this recycling we could not exist. Note without galaxies, the universe could expand as the cosmic accelerated expansion is caused by the mysterious dark energy ( not galaxies) and also note that heavy elements, we are made of, are not produced at the center of galaxies. However, it is true that the bulge (spheroid) of galaxies are populated by old stars and new stars tend to form in the spiral arm (disk). 6. If you could count stars at a rate of about one per second, how long would it take to count all the stars in the Milky Way Galaxy? Ans: galaxies/1galaxy/sec 1yr/( )=6369 years so it takes several thousand years to count 7. If we imagine the history of the universe compressed into one year, when dinosaurs became extinct? Ans: note dinosaurs become extinct about 65 million years ago. It is believed that an asteroid hit the Earth and caused mass extinction. This may not be far from the truth as the Sun has a vertical oscillation period of 87 million and the motion of the Sun might have perturbed the asteroid from its position in the solar system and forced it to impact the earth. At any rate, using 65 million years we get /( ) 365 days=1.7 days which means dinosaurs disappeared the day before yesterday (December 30) if today is the first year anniversary of the big bang. 8. Briefly explain what we mean by the statement The farther away we look in distance, the further back we look in time. Ans: It means that when we look at a distant object, we see it as it was some time in the past, rather than as it is now. This is because the light from the object can only travel at a constant and finite speed and therefore takes time to reach us. So astronomers can estimates the lower limit to the age of the universe by measuring the speed and distance to galaxies far away. You might have heard that astronomers at UCSC, using Hubble Space Telescope has found a galaxy 13.2 billion light years away. So if we are seeing a light emitted from this galaxy 13.2 billion years ago the age of universe cannot be lower than this if such object existed then. 9. Stars that are visible in the local sky on any clear night of the year, at any time of the night are know us what? Ans: Circumpolar stars. 10. If we have a new moon today, when we will have the next full moon? Ans: In about 2 weeks 3

4 11. Explain the observed apparent retrograde motion of planets. Ans: The Earth and other planets orbit the Sun at different speed and a planet appears to reverse its direction of motion when it overtakes the Earth or overtaken by the Earth. Think about when you are in the car things in front of you seems suddenly to whiz back when you surpass them. 12. What is a stellar parallax? Ans: It is the slight back-and-forth shifting of star s position as we view the star from different positions in the Earth s orbit around the Sun. The stellar parallax is a fraction of an arcsecond and not easy to measure. Since ancient Greeks did not have the capability we have now to measure parallax they were adamant that the Earth is stationary. 13. Suppose you have a friend who live in the South Africa and you see a crescent moon in your evening sky tonight. What will a friend in South Africa see tonight? Ans: he also sees a crescent moon. Remember the difference between your longitude does not affect the phase the Moon because the Earth rotates (it is a matter of who will see the moon first) but the difference in your latitude may slightly affect the percentage of the Moon you and your friend see since the orbit of the Moon is slightly inclined from ecliptic but this is a tiny effect to change the phase you see. 14. Approximately by how many degree does the Sun move along the ecliptic each day? In other words, by how many degrees the Earth move on its orbit around the Sun each day? Ans: One degree because the Earth takes a year to orbit the Sun and there are 365 days in a year and one orbit is 360 degree revolution. 15. What is the difference between a Sidereal Day and a Solar Day? Ans: A Sidereal day is a time of one earth rotation relative a fixed star while solar day is a time of rotation relative to the Sun. Due to the fact that the Sun moves by about one degree in a day, the solar day is longer by 4 minute (since the earth rotates 360 degree in 24 hours to rotates by one degree it need 24 hrs 60 min/1 hr)/360=4minutes). 17. It s 6 a.m and the Moon is at its highest point in your sky (crossing the meridian). What is the Moon s phase? Ans: The third quarter 18. If the Moon is a third quarter phase, when did it rise? Ans: Midnight 19. If the Moon rises around 3 am, what is its phase? Ans: Waning crescent 20. Suppose it is full moon. What phase of Earth would someone on the Moon see at this time? Ans: New Earth because it is night on half-earth that observer on the Moon is facing and because he is looking directly at Earth as the Moon and the Earth are perfectly aligned. Therefore, he barely see the day side of the Earth. 21. Why does not lunar eclipses happen once a month? Ans: Because the orbit of the Moon is inclined 5 degree from the ecliptic. 22. Do we have the four seasons if the distance from the Sun to the Earth is exactly 1 AU at all times as the Earth orbits the Sun? 4

5 Ans: Yes, the seasons are caused by the tilt of Earth s axis and distance of the Earth from the Sun is a negligible factor. 23. The observable universe is the same size today as it was a few billion years ago. Does this statement make sense? Ans: No, because the fact that the universe is expanding means the observable universe is growing with time. 24. For the galaxy at the center of the universe, nearly all galaxies must be moving away from it. Does this statement make sense? Ans: No, there is no such a thing as the center of the universe and all galaxies in the universe are equally special. 25. Stars beyond our solar system disappear during the day and reappear at night because they have orbital period of about 12 hours. Does this statement make sense? Ans: No, stars do not go away during the day. It is just that they are far away and are relatively faint to see during the day compared to the Sun. 26. Jupiter does not have seasons because of its lack of appreciable axis tilt. Does this statement make sense? Ans: yes, this is true. 27. The fact that the sky looks like a dome is an evidence that the universe is curved like sphere. Does this statement make sense? Ans: No, the universe is flat. The local sky looks like a dome because we see half of the full celestial sphere at any one time. Note that a celestial sphere a projection of object on the sphere and these object my in reality be very far apart. 28. The summer solstice is the day when the Northern Hemisphere gets the most sunlight. Does this statement make sense? Ans: Yes, I guess this is obvious. 29. I live in the United States, and during my first trip to Argentina I saw many constellations that I had never seen before. Does this statement make sense? Ans: Yes, constellations we see also depend on latitude as it depends on the Earth s motion around the Sun. 30. Lunar eclipses happen at new moon while Solar eclipses happen during the full moon. Does this statement make sense. Ans: No, the order is reversed. When a solar eclipse happens the Moon is between the Earth and the sun ( that is why it could block the sunlight) and therefore its phase is new moon. On the other hand, at Lunar eclipse, the sunlight is blocked by the Earth when it is between the Moon and the Sun.So the phase of the Moon is full during Lunar eclipse. 5

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