Introduction to Astronomy

Transcription

1 What is Astronomy? Introduction to Astronomy What are celestial bodies? Constellations there are 88 officially recognized constellations Eg. Ursa Major, Cassiopeia Big Dipper Ursa Minor Little Dipper Polaris Asterisms Not officially recognized star patterns Eg. the Big Dipper (part of the constellation Ursa Major) Ursa Major Models of Celestial Motion (1) Geocentric (Earth-Centered) Model Greek Philosopher Aristotle (400 BCE) believed that the earth was the center of the universe and all the other planets and the sun revolved around us. This view lasted 2000 years (2) Heliocentric (Sun-Centered) Model Copernicus (16 th Century) proposed that everything revolved around the sun, which was supported by Galileo 100 years later This is the accepted view we believe today We use the heliocentric model of the solar system as reference points for keeping time. The Earth revolves around the sun once every days. The Earth rotates around its axis once every hours.

2 Stars are giant glowing balls of gases in space. The Formation of Stars What is the most common element in the universe? Stars begin as large clouds of dust and gas (mostly hydrogen) known as a By gravity nebulae eventually collapse on themselves. At the core of this collapse, eventually there is so much pressure and such high temperatures that atoms begin to fuse. This is called NUCLEAR FUSION The process of nuclear fusion releases massive amounts of energy; this is what powers a star. What is the closest star to earth? Why it yellow? Red yellow blue 3000C The large the star, the hotter its core and the longer it lasts. Larger stars can also form more elements through fusion reactions 55000C As heavier elements form they move closer to the core of the star. Layers of different elements form throughout the star. A version of the periodic table indicating the origins of the elements. All elements above 103 (lawrencium) are also manmade and are not included.

3 Hertzsprung-Russell (HR) Diagrams Adapted from Las Cumbres Observatory Global Telescope Network. In the early 20th century, after investigating the effects of an object s temperature and of the color of its radiation, scientists reasoned that there should be a relationship between the temperature of a star and its luminosity (total energy emitted from a star). If all stars were alike, those with the same luminosity would have equal temperature and hotter stars would be brighter than cooler ones. In 1911, Ejnar Hertzsprung (Denmark) plotted a graph of star s magnitudes against their color. Independently in 1913, Henry Russell (USA), constructed a plot of stars magnitudes against their spectral class, confirming that indeed, there did seem to be some sort of relationship between a star s luminosity and its temperature, and the stars fell into distinct groups. Such a plot was thereafter named the Hetzsprung-Russell or H-R diagrams. A star on a HR diagram is represented by a dot. Since a large number of stars are usually represented on a HR diagram, there are a large number of dots on the diagram. The y-axis on a HR diagram represents the star s luminosity and the x- axis represents the temperature of the star, both on a logarithmic scale. Main sequence The main sequence is a band which stretches from the bottom right of the HR diagram up to the top left, hence it goes from cooler, dimmer stars up to brighter, hotter ones. Most stars, including our Sun spend most of their lives on the main sequence as they fuse their Hydrogen into Helium in their cores. Red Giant Branch Once intermediate mass stars like the sun have fused all their hydrogen into helium in their core, they evolve off the main sequence into the Red Giants area (which astronomers call the Red Giant branch or RGB). At this point, fusion of the hydrogen shell surrounding the core of the star begins. The outer shell of the star inflates dramatically and cools. Red Supergiants When the hydrogen shell burning is finished, the shell of helium begins fusing into heavier elements such as carbon and oxygen. As this happens, the star moves into the red supergiants region of the HR diagram. White Dwarfs Once all the Helium has been fused into other elements, the outer layers of the star are ejected outwards into what is known as a planetary nebula. The exposed, leftover core of the star (made up of mostly carbon and oxygen), is a white dwarf. The white dwarf gradually becomes fainter and cooler as fusion slows. Evolution from the red supergiant area to the white dwarf area happens very quickly in comparison to how long the star stayed on the main sequence. Blue Giants A blue giant is a massive star that has exhausted the hydrogen fuel in its core and left the main sequence. Blue giants have a surface temperature of around C and a luminosity some times that of the Sun. As they grow older they expand and cool, eventually becoming red giants, or continuing fusion into a more luminous or massive star. Since they are so hot, their expected life is very short. Theories predict most of that these massive starts will end their lives as supernovae. A supernova is a stellar explosion that briefly outshines an entire galaxy before fading from view over several weeks or months.

4 Hertzsprung-Russell (HR) Diagram

5 Life Cycle of Stars

6 The Formation of The Solar System Planets are massive, round, spinning balls of rock and/or gas that orbit stars (a) Name all of the planets in order from closest to farthest from the sun. Divide the inner planets from the outer planets by showing the location of the asteroid belt. NOTE: THIS DIAGRAM IS NOT TO SCALE (b) What are the main differences between the inner planets and the outer planets? Inner Planets Outer Planets (c) What is found in our solar system between Mars and Jupiter? (d) Why is Pluto now considered to be a dwarf planet?

7 (e) How do planets form? (add notes next to the diagrams below to answer this question) d) Distances in Space are HUGE. How can we measure them? Astronomical Units (AU): Light Years 1 light-year = meters = AU

8 Planetary Summary Sheet Planet Name Distance from Sun Size (diameter) Temp. Time of Orbit Moons Atmosphere Description (main features)

9 Define the main characteristics of each of the following celestial bodies. (Pg ) Comet Asteroid Meteor Meteorite 1) Describe the main differences between comets and asteroids. 2) Describe the process of changing from an asteroid, to a meteor to a meteorite. 3) Why do comets have tails?

10 Important Background: MOTION OF THE EARTH The Reason for the Seasons The Earth completes one full rotation in the course of one day. It rotates around its axis which is an imaginary line running through the center of the Earth from pole to pole. This takes about 24 hours (23 hours 56 minutes and 4 seconds) Since the earth is a sphere, at any given time, half of it is in the light of the sun and half of it is in darkness. When we pass through these regions as we rotate, we experience day and night. SUN The tilt of the Earth is fixed at a 23.5 degree angle relative to its path of orbit around the Sun. The North pole is always pointed directly at Polaris (often called the North Star ) All of the stars in the sky appear to rotate counter clockwise around Polaris One year corresponds to the time it takes for the Earth to complete one revolution around the Sun This takes about 365 ¼ days (in other words, the Earth rotates times until it returns to the same spot in its orbit) Since the tilt of the axis is constant relative to the universe rather than to the Sun, at different times of the year the north pole is either angled more towards or away from the Sun It is the TILT of the Earth that causes the Seasons

11 Direct vs Indirect Sunlight a a b b Solstices and Day Length N N Sketch a person receiving 24 hours of darkness in the high arctic during winter Sketch a person receiving 24 hours of darkness in Antarctica during winter

12

13 Eclipses To understand how eclipses work, we must first understand how shadows are cast by celestial bodies. The darkest part of a shadow where no light reaches is called the umbra. A partial shadow is called a penumbra. Celestial bodies passing into shadows produce the effects of an eclipse. A total eclipse occurs when the celestial body passes through the umbra. A partial eclipse occurs when the celestial body passes through the penumbra. Use a rule to draw lines representing rays of light encompassing the maximal range of light from the sun hitting a celestial body. Shade in and label the umbra and the penumbra. SUN Celestial body Solar Eclipse A solar eclipse occurs when the earth passes into the shadow of the moon. Characteristics: Lasts only a brief period of time Total eclipse only visible in a narrow location Looking directly at a solar eclipse for too long can cause retinal damage Lunar Eclipse A lunar eclipse occurs when the moon passes into the shadow of the earth. Characteristics: Lasts longer than a solar eclipse (Earth s shadow is larger than moon s shadow) A lunar eclipse will be seen simultaneously anywhere on Earth where the moon is visible. During an eclipse the moon appears to change to a brown colour as it passes through the Earth s shadow.

14 Why do we not experience a solar or lunar eclipse every month as the moon revolves around the earth? The moon does not orbit in the same plane as the Earth s orbit around the sun. All three celestial bodies must line up perfectly for an eclipse to be visible. Solar eclipses occur two or three times a year and last as long as 7.5 minutes. Any one point on Earth may on the average experience no more than one total solar eclipse every 3-4 hundred years. Lunar eclipses occur many times per year (3-6 times) and can last as long as an 1.75 hours. PRACTICE Use a ruler to draw light rays to find the shadow cast in each eclipse. Label the umbra and penumbra in each.