Class of oc 2020 21 ch.1
- 1. PY 1551.2 ASTRONOMY AND ASTROPHYSICS
- 2. Unit I: Introduction to Astronomy Humans have long gazed toward the heavens, searching to put meaning and order to the universe around them. Although the movement of constellations — patterns imprinted on the night sky — were the easiest to track, other celestial events such as eclipses and the motion of planets were also charted and predicted.
- 3. • What is astronomy Astronomy is the study of the sun, moon, stars, planets, comets, gas, galaxies, dust and other non-Earthly bodies and phenomena NASA defines astronomy as simple "the study of stars, planets and space." Astronomy and astrology were historically associated, but astrology is not a science and is no longer recognized as having anything to do with astronomy
- 4. Astronomy is broken down into a number of subfields, allowing scientists to specialize in particular objects and phenomena 1. Planetary astronomers (also called planetary scientists) focus on the growth, evolution, and death of planets. While most study the worlds inside the solar system, some use the growing body of evidence about planets around other stars to hypothesize what they might be like. planetary science "is a cross- discipline field including aspects of astronomy, atmospheric science, geology, space physics, biology and chemistry."
- 5. 2. Stellar astronomers turn their eyes to the stars, including the black holes, nebulae, white dwarfs and supernova that survive stellar deaths. "The focus of stellar astronomy is on the physical and chemical processes that occur in the universe.“ 3. Solar astronomers spend their time analyzing a single star — our sun."The quantity and quality of light from the sun varies on time scales from milli-seconds to billions of years."Understanding those changes can help scientists recognize how Earth is affected.
- 6. The sun also helps us to understand how other stars work, as it is the only star close enough to reveal details about its surface. 4. Galactic astronomers study our galaxy, the Milky Way, while extragalactic astronomers peer outside of it to determine how these collections of stars form, change, and die. "Establishing patterns in the distribution, composition, and physical conditions of stars and gas traces the history of our evolving home galaxy."
- 7. 5. Cosmologists focus on the universe in its entirety, from its violent birth in the Big Bang to its present evolution, all the way to its eventual death. Astronomy is often (not always) about very concrete, observable things, whereas cosmology typically involves large-scale properties of the universe and esoteric, invisible and sometimes purely theoretical things like string theory, dark matter and dark energy, and the notion of multiple universes.
- 8. Astronomical observers rely on different wavelengths of the electromagnetic spectrum (from radio waves to visible light and on up to X-rays and gamma-rays) to study the wide span of objects in the universe. The first telescopes focused on simple optical studies of what could be seen with the naked eye, and many telescopes continue that today. Different telescopes are necessary to study the various wavelengths.
- 9. More energetic radiation, with shorter wavelengths, appears in the form of ultraviolet, X-ray, and gamma-ray wavelengths, while less energetic objects emit longer-wavelength infrared and radio waves. The celestial sphere and stellar magnitudes Looking up at the heavens on a clear night, we can imagine that the stars are located on the inside of a sphere, called the celestial sphere, whose centre is the centre of the Earth.
- 10. The constellations As an aid to remembering the stars in the night sky, the ancient astronomers grouped them into constellations; representing men and women such as Orion, the Hunter, and Cassiopeia, mother of Andromeda, animals and birds such as Taurus the Bull and Cygnus the Swan and inanimate objects such as Lyra, the Lyre. There is no real significance in these stellar groupings – stars are essentially seen in random locations in the sky – though some patterns of bright stars, such as the stars of the ‘Plough’ (or ‘Big Dipper’) in Ursa Major, the Great Bear, result from their birth together in a single cloud of dust and gas. The chart in Figure 1.4 shows the brighter stars that make up the constellation of Ursa Major.
- 12. The brightest stars in the constellation (linked by thicker lines) form what in the UK is called ‘The Plough’ and in the USA ‘The Big Dipper’, so called after the ladle used by farmers’ wives to give soup to the farmhands at lunchtime. On star charts the brighter stars are delineated by using larger diameter circles which approximates to how stars appear on photographic images. The grid lines define the positions of the stars on the celestial sphere as will be described below.
- 13. Stellar Magnitudes The early astronomers recorded the positions of the stars on the celestial sphere and their observed brightness. The first known catalogue of stars was made by the Greek astronomer Hipparchos in about 130–160 BC. The stars in his catalogue were added to by Ptolomy and published in 150 AD in a famous work called the Almagest whose catalogue listed 1028 stars. Hipparchos had grouped the stars visible with the unaided eye into six magnitude groups with the brightest termed 1st magnitude and the faintest, 6th magnitude.
- 14. When accurate measurements of stellar brightness were made in the nineteenth century it became apparent that, on average, the stars of a given magnitude were approximately 2.5 times brighter than those of the next fainter magnitude and that 1st magnitude stars were about 100 times brighter than the 6th magnitude stars. (The fact that each magnitude difference showed the same brightness ratio is indicative of the fact that the human eye has a logarithmic rather than linear response to light.)
- 15. In 1854, Norman Pogson at Oxford put the magnitude scale on a quantitative basis by defining a five magnitude difference (i.e., between 1st and 6th magnitudes) to be a brightness ratio of precisely 100. If we define the brightness ratio of one magnitude difference as R, then a 5th magnitude star will be R times brighter than a 6th magnitude star. It follows that a 4th magnitude star will be R×R times brighter than a 6th magnitude star and a 1st magnitude star will be R×R×R×R×R brighter than a 6th magnitude star. However, by Pogson’s definition, this must equal 100 so R must be the 5th root of 100 which is 2.512.
- 16. The brightness ratio between two stars whose apparent magnitude differs by one magnitude is 2.512. Having defined the scale, it was necessary to give it a reference point. He initially used Polaris as the reference star, but this was later found to be a variable star and so Vega became the reference point with its magnitude defined to be zero. (Today, a more complex method is used to define the reference point.)
- 17. Apparent magnitudes It should be noted that the observed magnitude of a star tells us nothing about its intrinsic brightness. A star that appears bright in the sky could either be a faint star that happens to be very close to our Sun or a far brighter star at a greater distance. As a result, these magnitudes are termed apparent magnitudes. The nominal apparent magnitudes relate to the brightness as observed with instruments having the same wavelength response as the human eye.
- 18. one can also measure the apparent magnitudes as observed in specific wavebands, such as red or blue, and such measurements can tell us about the colour of a star. Some stars and other celestial bodies, such as the Sun, Moon and planets are much brighter than Vega and so can have negative apparent magnitudes. Magnitudes can also have fractional parts as, for example, Sirius which has a magnitude of -1.5
- 19. The celestial coordinate system The early star catalogues located the positions of the stars on the celestial sphere using a slightly different coordinate system than we do now. The modern coordinate system is analogous to the way in which we define positions on the surface of the Earth and uses the orientation of the Earth in space as its basis. The Earth’s rotation axis is extended up and down to the points where it reaches our imaginary celestial sphere.
- 21. The point where the axis meets the sphere directly above the North Pole is called the North Celestial Pole and that below the South Pole is the South Celestial Pole. If the Earth’s equator is extended outwards it will cut the celestial sphere into two – into the northern and southern hemispheres – forming the Celestial Equator
- 22. There is one path around the celestial sphere that is of great importance: that of our Sun. If the Earth’s rotation axis was at right angles to the plane of its orbit around the Sun, the Sun’s path would trace out the Celestial Equator but, as the axis of the Earth’s rotation is inclined to its orbital plane by an angle of 23.5°, the path of the Sun is a great circle, called the ecliptic, which is inclined by 23.5° to the Celestial Equator. The Sun spends half the year in the southern half of the celestial sphere and the other half in the northern.
- 23. Its path thus crosses the Celestial Equator twice every year: once at the vernal equinox, on March 20 or 21, as it comes into the northern hemisphere and 6 months later when, at the autumnal equinox on September 22 or 23, it returns to the southern hemisphere. Just as a location on the Earth’s surface has a ‘latitude’, defined as its angular distance from the equator towards the poles, so a star has a “declination” (Dec) given as an angle which is either positive (in the northern hemisphere) or negative(in the southern hemisphere). The ‘Pole Star’ in the northern sky is close to the North Celestial Pole at close to 90ᵒ declination and the region at the South Celestial Pole (where there is no bright star) is at -90ᵒ declination.
- 24. The second coordinate proves to be rather more difficult. On the Earth we define the position of a location round the Earth by its longitude. However, there has to be some arbitrary zero of longitude. It was sensible that the zero of longitude, called the Prime Meridian, should pass through a major observatory and that honour finally fell to the Royal Greenwich Observatory in London. As referred to above, the path of the Sun gives two defined points along the Celestial Equator that might sensibly be used as the zero of Right Ascension (RA)– the points where the ecliptic crosses the Celestial Equator at the vernal and autumnal equinoxes.
- 25. The point where the Sun moves into the northern hemisphere was chosen and was given the name ‘The first point of Aries’ as this was the constellation in which it lay. Star positions are measured eastwards around the celestial sphere from the first point in Aries to give the star’s RA. However, for reasons that will become apparent when we describe how star positions are measured, RA is not measured in degrees but in time, with 24 h equivalent to 360°. Hence, the celestial sphere is split into 24 segments each of 1 h and equivalent to 15° around the Celestial Equator.
- 26. Precession Of Earth’s axis Should you locate the point where the Sun crosses the ecliptic at the vernal equinox on a star chart (with position: RA 0:00 h, Dec 0.0°), you might be surprised to find that it is not in Aries, but in the adjacent constellation Pisces. This is the result of the precession of the Earth’s rotation axis in just the same way that the axis of rotation of a spinning top or gyroscope is seen to precess.
- 27. The precession rate is slow; one rotation every ∼26 000 years, but its effect over the centuries is to change the positions of stars as measured with the co-ordinate system described above, which is fixed to the Earth. Consequently, a star chart is only valid for one specific date. Current star charts show the positions of stars as they were at the start of the millennium and will state ‘Epoch 2000’ in their titles. One result of precession is that the Pole Star is only close to the North Celestial Pole at this particular moment in time in the precession cycle
- 28. In ∼12 000 years, the bright star Vega will be near the North Celestial Pole instead (though by no means as close). It also means that constellations currently not observable from the UK will become visible above the southern horizon. Interestingly, it is stars in the part of the sky that was visible to ancient astronomers and which were thus included in the constellations that enable us to estimate not only the time but also the latitude from which the constellations were delineated and named.
- 29. A region of about 36° radius in the southern sky did not contain any of the original 48 constellations implying that this region was invisible to those who mapped the sky. This is precisely the region that would have been invisible to those living at a latitude of 36° north. Due to precession, the stars that would be hidden from view in this region will vary with time, and this enables us to give a date, about 2600–2900 BC, when the constellations were delineated.
- 30. Thank You