Back to the Index of I2AO (Introduction to Astronomy Online)
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PART 1: ASTRONOMY BASICS
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The name "astronomy" derives from astrum (star) and nomen (name). In ancient times, astronomers named and mapped the stars. The modern science of astronomy investigates what things are made from, how they work, and why they look the way they do. Astronomers try to understand what it is they see in the sky. Astronomers are scientists.
A non-astronomer thinks of an object as something solid, perhaps something that they can hold in their hand. Astronomers study "objects" in the sky or space, also known as celestial objects. Some of these are solid, some are made from gas and some are collections of objects or regions that are bound together by gravity. Types of objects include:
3. How do we see these objects?
Figure 1 below: This is the summit of Mauna Kea, on Hawai'i (the Big Island of the Hawai'ian Islands).
From left to right: 24" (very small dome on left), University of Hawaii 88", Gemini 8-meter, Canada-France Hawaii Telescope (CFHT), James Clerk Maxwell Telescope (dimly lit in shadow behind CFHT), NASA Infrared Telescope Facility (silver dome in front of twin Keck domes), Keck I&II (large twin white domes), Subaru (silver structure in Keck I shadow).
Photo Credit: Copyright 1999, Neelon Crawford - Polar Fine Arts, courtesy of Gemini Observatory and National Science Foundation
Figure 2 below: This exterior view of Mauna Kea highlights the Gemini North Observatory which is the largest dome near the center of the image. This image was taken shortly after a significant snowfall in early 1999 before skiers hit the slopes! The 360-degree panoramic image is made from 18 images stitched together using Apple QuickTimeVR technology. This image is part of a QuickTimeVR series that can be accessed on the www at: www.gemini.edu/public/movie.html. This image is from a complete CD-ROM/WWW virtual tour of Gemini.
Photo Credit: Courtesy Gemini Observatory
Figure 3 below: The Very Large Telescope (VLT) comprising four 8-metre mirrors in separate enclosures. This one is located in Chile.
Photo Credit: Courtesy Ángel Rafael López-Sánchez
Space is even better, e.g. the Hubble Space Telescope orbits the Earth.
For amateurs, a dark back yard or open field away from city lights is suitable.
5. When is the best time to look?
Figure 4 below: Orion, a constellation that is visible in Australia on summer evenings, appears on the left-hand-side of this image, to the right of the tree. The star pattern (asterism) known as the "saucepan" is part of this constellation. Stars in this image look like little streaks because the image was made using an exposure of five minutes or so, in which time the Earth has spun a little on its axis, making the stars appear to move during the exposure. This effect is known as star trailing.
Photo Credit: Lesa Moore
Figure 5 below: Scorpius, a constellation that is easily seen in the wintertime from Australia, lies above the clothesline in this image. Jupiter is also visible in the lower left of the photo, below the "head" of the scorpion.
Photo Credit: Lesa Moore
Figure 6 below: This photograph of the Sun, taken on December 19, 1973, during the third and final manned Skylab mission, shows one of the most spectacular prominences ever recorded, spanning more than 588,000 kilometres (365,000 miles) across the solar surface. The loop prominence gives the distinct impression of a twisted sheet of gas in the process of unwinding itself. In this photograph the solar poles are distinguished by a relative absence of supergranulation network, and a much darker tone than the central portions of the disk. Several active regions are seen on the eastern side of the disk. The photograph was taken in the light of ionized helium by the extreme ultraviolet spectroheliograph instrument of the U.S. Naval Research Laboratory. (Wikipedia Caption)
Image Credit: NASA
A celestial body that:
(a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
(c) has cleared the neighbourhood around its orbit.
9. Planets of the Solar System
Figure 7 below: The illustration below shows the sizes of the Sun, planets and dwarf planets approximately to scale. Separations/distances are not to scale, but the order is correct, based on the average distance of each body from the Sun. Note that orbits are elliptical and, at certain times, Pluto orbits closer to the Sun than does Neptune.
Image credit: NASA
Figure 8 below: Orbits of the "gas giant" planets, Jupiter, Saturn, Uranus and Neptune, and the dwarf planets, Pluto and Eris. The Kuiper belt is a region where distant asteroids and dwarf planets orbit.
Image credit: NASA
Figure 9 below: The Moon. This composite image was created by putting a first-quarter Moon photo and a third-quarter Moon photo together. Because the Sun's rays are at a lower angle on both halves, the topography of the Moon's surface is much clearer. This sort of composite makes the lunar features near the central meridian stand out in greater detail.
Image credit: Lick Observatory
Figure 10 below: Composite image of Jupiter and its four brightest and largest moons. Jupiter is in the background, with the Great Red Spot visible on the lower half of the planet. The moons are Io to the left of Jupiter, Europa in front of the planet, Ganymede seen in full at the bottom of the image and Callisto in the foreground at bottom right.
Image credit: NASA
11. Other things in the Solar System
Asteroids may be found in many locations in the Solar System.
Figure 11 below: The asteroids of the inner Solar System. Main-belt asteroids are shown in white. The blue rings are planetary orbits. The Trojans, Greeks and Hildas are labelled in the diagram.
Image Credit: Wikipedia (public domain)
Figure 12 below: Asteroid trail on a star image. The telescope tracks the stars, which appear as points of light. The asteroid moves with respect to the background stars so, on a long exposure (30 seconds in this example), it makes a trail of light on the detector.
Image Credit: Buzzi & Luppi, Schiaparelli Astronomical Observatory
Comets orbit the Sun with periods that range from less than 100 years (short-period comets) to thousands of years (long-period comets).
Meteroids, Meteors and Meteorites
Meteoroids are particles that orbit the Sun, chucks of rock and broken-up asteroids that are generally small and have not yet been swept up by the planets and other large bodies. Some enter Earth's atmosphere and burn up as meteors. Meteors are seen as rapid, visible streaks of light in the night sky, the light being caused by excitation of the atmosphere as the object falls through it. Very large meteors may also leave persistent and, sometimes, smoky, trails. Meteors are also referred to as "falling stars", even though they are not stars. Larger meteoroids that don't burn up, but make it through to the ground to land as rocks, are called meteorites.
Figure 14 below: A diagram showing blackbody radiation curves (the amount of light given off at each colour of the spectrum) for a hot blue star (15000 kelvins surface temperature), a Sun-like star (6000 K) and a cool red star (3000 K). For the blue star, more blue light is emitted than red (i.e. the curve is higher on the blue side of the colours), so the star looks blue. It is the reverse for the red star. A 6000 degree star (e.g. the Sun) looks white because roughly equal amounts of all colours are given off and they blend together to produce white light. The fourth curve (the small black one in the diagram) is that of a human, who emits most energy in the infrared (non-visible) part of the spectrum, i.e. humans radiate heat, not light.
Image Credit: Diagram by Lesa Moore
Figure 15 below: This is a progressively defocused image of the constellation, Orion. The red supergiant, Betelgeuse, appears yellow at the top, left. The three bright stars in a row across the middle are classed as hot blue stars. The pinkish region in the bottom half, left of centre, is emission from hydrogen gas in a nebula (produced by a different mechanism entirely, and not blackbody radiation).
Image Credit: David Malin
Figure 16 below: A colour temperature chart. Note that the colours are more pastel than in Figure 13. A "red" star would really look more of a pastel orange or yellow, as in Figure 14, rather than a true red.
Image Credit: Wikipedia (public domain). Original caption: CIE xy 1931 chromaticity diagram including the Planckian Locus. The Planckian locus is the path that a black body color will take through the diagram as the black body temperature changes. Lines crossing the locus indicate lines of constant correlated color temperature. Monochromatic wavelengths are shown in blue in units of nanometers.
13. Examples of Deep Sky Objects
The following images show examples of these objects.
Figure 17 below: An open cluster, the Jewel Box. Stars in an open cluster all form from the same cloud of gas and dust. There may be a handful up to a couple of hundred stars in an open cluster. The stars are loosely held by gravity and may drift apart from each other over time.
Image Credit: European Southern Observatory
Figure 18 below: A globular cluster, Omega Centauri. Globular clusters contain thousands up to, perhaps, a million stars that are tightly gravitationally bound together.
Image Credit: Australian Astronomical Observatory
Figure 19 below: A spiral galaxy, M74. Galaxies like this may contain a couple of hundred billion stars.
Image Credit: Ángel Rafael López-Sánchez
Figure 20 below: An elliptical galaxy, M87. Elliptical galaxies range greatly in size. M87 is a large elliptical galaxy in the Virgo Cluster.
Image Credit: Canada-France-Hawaii Telescope, J.-C. Cuillandre (CFHT), Coelum
Figure 21 below: An irregular galaxy, the Large Magellanic Cloud. Irregular galaxies do not show features like the arms of a spiral galaxy, nor the football-shape of an ellliptical galaxy. Irregulars are small-sized galaxies.
Image Credit: Australian Astronomical Observatory. Original caption: The Large Magellanic Cloud (LMC) is the nearest galaxy to the Milky Way but less than one tenth as massive; even so it contains the equivalent of over ten billion solar masses of material in the form of stars, gas and dust. The LMC is at a distance of 170,000 light years and is visible to the unaided eye from southern latitudes, with an apperance rather like a detached piece of the Milky Way, in the otherwise barren constellation of Dorado.
Figure 22 below: An emission nebula, the Tarantula Nebula in the Large Magellanic Cloud. Stars form in this type of nebula, which is emitting light from hydrogen gas that is excited by hot, young stars. An emission nebula is a stellar nursery.
Image Credit: Joe Cauchi (ASNSW)
Figure 23 below: A reflection nebula, the Trifid Nebula (M20). This nebula is, in fact, two types of nebulae in the one object. On the right, the characteristic pinkish looking region is an emission nebula. On the left, the blue region is reflection nebulosity. A reflection nebula shines simply by light reflecting off dust in space.
Image Credit: Kevin Cooper (ASNSW)
Figure 24 below: A planetary nebula, Abell 70. A planetary nebula is the left-over expanded atmosphere of a star like the Sun that has reached the end of its life. The star, after exhausting its fuel, collapses to become a white dwarf (about the size of the Earth). In this case, the white dwarf is clearly seen at the centre of the planetary nebula.
Image Credit: Brent Miszalski (South Africa Large Telescope).
Figure 25 below: A dark nebula, the Horsehead Nebula. Dark nebulae are patches of dust in space that absorb the light from bright objects behind them. A dark nebula will appear dark against a bright stellar background or, as in this case, an emission nebula.
Image Credit: Trevor Gerdes (ASNSW)
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Author: Lesa Moore, 26th July 2017. Minor revisions 28th January 2018.
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