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Astronomy Essay - John Coleman DACE 201111
GC235 Introduction to astronomy DACE 2011
Assessment Essay by John Coleman 0709096 (5017 words)
mailto:[email protected]
Notes on an Introduction to astronomy
Notes on an Introduction to astronomy
1
1 Introduction
2
2 The naming of parts
3
3 Starry, starry nights…
4
3.1
Diurnal motion..............................................4
3.2
Constellations ...............................................6
3.3
Clusters.........................................................7
3.4
Nebulae ........................................................7
4 … and Free Open Source Software for a rainy night
4.1
Stellarium [5] and Celestia [6] .....................8
4.2
hygxyz.csv [8] and exoplanets.csv [9] ...........9
4.3
OpenJUMP [10]............................................9
5 The Solar System
11
5.1
The Sun ...................................................... 12
5.2
The Planets [16].......................................... 13
6 Stars
18
6.1
Spectrometry............................................... 18
6.2
Measuring distance to stars ......................... 19
6.3
The Hertzprung-Russell (H-R) Diagram...... 22
7 References
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Astronomy Essay - John Coleman DACE 201111
1 Introduction
My DACE philosophy course on The idea of religion was cancelled at the last
minute due to lack of interest – so I switched to Introduction to astronomy.
Hitherto, I have completed four DACE courses on various aspects of philosophy
and three on introductory geology. Introduction to astronomy fits into that
syllabus quite well actually; there is a common theme of Natural Philosophy.
Natural Philosophy is a phrase that I like, not only because it includes my degree
subject of physics, but also because it implies a rational approach and natural
science.
Previously I have hardly studied astronomy at all, beyond a casual interest in
constellation spotting and a memorable inspection of Jupiter’s moons by
telescope on a starry night at a school event. I have bored my family
occasionally by trotting out the same old ‘amazing facts’ about the size of
Betelgeuse and how the Moon keeps its far side hidden from us. Once upon a
time I studied relativity and nuclear physics at university, then moved on to
(briefly) teach Newton’s laws of motion and gravity at a secondary school in
Yorkshire. I moved on again, into software engineering via medical physics and
computer tomography; working with Geographical Information Systems, and
(inevitably) database design. Now I am semi-retired and there’s time to keep my
brain in gear by filling in some of the many gaps in my knowledge.
Andrew Conway explained that Introduction to astronomy is an ACE course and
that an assessment option is available, simply by providing a suitable essay or
body of related work. Without a list of possible essay subjects I found it hard to
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begin, but finally settled on presenting these notes as a kind of retrospective blog.
I know from previous experience that attempting to set out what I have been told
will help me learn it much better. I can’t cover the whole course in one essay so I
will draw the line after The Hertzprung-Russell Diagram.
References are by digits in square brackets, relating to numbered end-notes,
which are hyperlinks to items in the Glasgow University Library catalogue, or
remote WWW resources. Hyperlinks won’t work on printed hard-copy, so the
references at least are best accessed on an Internet-enabled PC via the .pdf file
and Adobe Reader. Most of these notes are based on Andrew Conway’s classes,
supplemented by references to his recommended books, Inquest of the
universe[1], and Universe[2], which I have found very useful.
I have tried not to plagiarise too much but a few phrases from the reference works
may be repeated verbatim, they are italicised. I don’t like to paraphrase just for
the sake of it and I am far from expert at astronomy.
2 The naming of parts
Andrew began with a glossary of astronomical terms – he chooses his words carefully:
star
massive gas cloud
planet
less massive cloud, of gas or rock and ice
constellation
group of stars, not necessarily closely co-located in space
cluster
group of stars co-located in relatively small area of space
nebula
gas cloud
galaxy
enormous collection of stars
universe
everything
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3
Starry, starry nights…
…are quite rare in Glasgow, often due to overcast weather and always due to
urban light pollution. People in remoter places on cloudless nights enjoy super
night skies. Night after night they can observe stars rising above the eastern
horizon and setting below the western horizon, while remaining fixed in position
relative to the neighbouring stars in their constellation.
3.1 Diurnal motion
It is not quite so easy to observe stars moving across the sky during the night, but
a simple long exposure photograph demonstrates it clearly: see Figure 1.
Figure 1: The Heilig-Kreuz-Kirche in Bad Tölz, Germany, long time exposure. [3]
Some stars never rise or set, they seem to follow a circular path around the pole
star, Polaris (in the Northern hemisphere – there is no similarly convenient polar
star in the southern hemisphere). These stars are called circumpolar and the list of
circumpolar stars is determined by the viewer’s latitude. In Glasgow, at N 55° 52'
12.00" all the stars within ~34° (i.e. 90-56) of Polaris are circumpolar, including
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Vega. These stars just appear after sunset, when sunlight scattered through the
atmosphere decreases sufficiently for them to become visible.
Andrew Conway casually mentioned that any given star rises 4 minutes earlier on
each successive night – leaving me trying to get my head round why that should
be! It turns out that the earth’s period of rotation with respect to the fixed stars,
the sidereal day, is actually 23 hours 56 minutes but with respect to the sun, the
solar day, it is 24 hours exactly. The extra 4 minutes are because during each
rotation, the earth moves relative to the sun as it continues its annual orbit. As it
does so the earth’s nighttime hemisphere includes progressive zodiacal
constellations, while the sun is ‘in’ the diametrically opposite constellation. Figure
2 show the apparent ecliptic path of the sun through the zodiacal constellations.
Figure 2: Ecliptic path [4]
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It is easy to see why the ancient astronomers adopted a geocentric model, with
the earth at the centre of an immense celestial sphere and the fixed stars
embedded in its surface. They easily explained the stars’ diurnal motion by
proposing that the celestial sphere rotates around the earth. But how to explain
the apparent movement of the sun through the other stars? Ancient Greek
astronomers were also aware of other heavenly bodies, apart from the Sun and
Moon, which were clearly not fixed to any celestial sphere - namely Mercury,
Venus, Mars, Jupiter and Saturn. These are the planets, named from the Greek
word for wanderer because they seem to wander among the stars. They are very
difficult to explain on a geocentric basis, although Ptolemy tried hard in the 2nd
century A.D. His Ptolemaic system was finally blown away by Copernicus
(1473-1543) who developed a comprehensive heliocentric model, and all without
the use of a telescope – which was not invented until the 17th century.
3.2 Constellations
Only about 6000 stars are visible by eye unaided, and half of them are below the
horizon at any one time. The ancient astronomers only named constellations of
bright stars that were familiar to them, so many stars would not be associated
with a particular constellation. Modern astronomers divide the whole sky into 88
touching regions, each identified as a constellation – thus all stars (and clusters
and nebulae etc) relate to a constellation, but it might not look much like a great
bear or a hunter. There are five circumpolar constellations in northern skies:
Cassiopeia, Cepheus, Draco, Ursa Major and Ursa Minor. The others are more
seasonal e.g. Orion is prominent in the winter and Scorpius in the summer (when
the nights are shorter).
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Many people recognise Betelgeuse in Orion as an orange or reddish star, and
Rigel as a bluish colour but reporting colour by eye is notoriously subjective.
Modern spectroscopy is utterly objective and very accurate. Andrew proposed
that astronomy + spectroscopy = astro-physics.
3.3 Clusters
Star clusters are important because they are closely related to each other in space.
They probably all formed at about the same time from the same giant molecular
cloud, so they should have a similar chemical composition. Because they are
relatively closely co-located their relative apparent magnitude is (for a change)
directly proportional to their relative absolute magnitude – there is no need to
correct for vast differences in distance. Studying star clusters has been very
useful in pursuit of understanding how stars are formed. The Pleiades (M45) is
an open cluster in the constellation of Taurus, it includes ~500 stars, 6 of which
can be seen with the naked eye. The Great Globular Cluster in Hercules (M13) is
a globular cluster, with about 300,000 stars! Globular clusters are spherical
groups of up to hundreds of thousands of stars, found primarily in the halo of the
Galaxy.
3.4 Nebulae
Orion is also ‘home’ to two important nebulae, the Great Orion Nebula and the
Horsehead Nebula. Nebulae are giant clouds of gas, illuminated by light from the
stars within them. They may be massive but they are huge, less densely
populated than a laboratory vacuum! The Great Orion Nebula is a diffuse nebula
situated south of Orion's Belt. It is one of the brightest nebulae and is visible to
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the naked eye in the night sky, a stellar ‘nursery’ where stars are formed from the
nebula’s gas. The Horsehead Nebula comprises clouds of interstellar dust which
block light from a bright nebula behind. Nebulae may be visible by emission
(e.g. Great Orion), reflection, or occlusion (e.g. Horsehead).
There are also planetary nebulae, so called because they were first thought to be
similar in colour to Uranus and Neptune. They have no other similarity with
planets. The Ring Nebula (M57) is a famous planetary nebula in Lyra, located
south of the bright star Vega. It is a good starter target for a new telescope.
4 … and Free Open Source Software for a rainy night
4.1 Stellarium [5] and Celestia [6]
Andrew explained to us that on a typical, 10 session, Introduction to Astronomy
course in Glasgow there might be one good starry night. We are still hoping.
But meantime he introduced us to some great free software, and has used it to
very good effect in the classroom (Celestia), and even better in the planetarium
(stellarium).
I have installed them both and they are very good. Stellarium is the amateur
astronomer’s equivalent of Google Streetview – no need to go out on a cold rainy
night, just use stellarium to inspect what you could see except for the overcast
and light pollution. Celestia is a simulator for aspirant astronauts who want to
jump to light-speed and beyond. Star-trek meets 3D modelling, complete with
warp-drive sound effects. Quite good fun but astronaut training is required – I
like stellarium better, especially as I don’t have an android smart phone to run
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Google Sky Map[7]. Celestia is very good though for giving an impression of the
vastness of the universe.
4.2 hygxyz.csv [8] and exoplanets.csv [9]
Not software exactly, but free machine-readable catalogues of stars and
exoplanets respectively. I downloaded hygxyz.csv and inserted the data into my
PostGIS database, in a spatially enabled version of the Postgres database server
(both free as well). That installs records for 119,617 stars, with most of the
important parameters: HIP, HD, HR, Gliese catalogue IDs; proper and BF names;
RA and Dec; Distance; Mag; AbsMag; spectral type; Cartesian coordinates;
Cartesian velocities. Now I can analyse the data with any database query I
choose e.g. SELECT * FROM Star WHERE DEC > 34 to find the stars which
are circumpolar at Glasgow.
4.3 OpenJUMP [10]
A spatially enabled database is no fun unless you can make maps from its
content. I installed OpenJUMP, which can connect to PostGIS. Now I can use
PostGIS to make stereographic projections (or other projections) of selected parts
of the celestial sphere, and use OpenJUMP to inspect the result. After some
considerable effort I achieved what Ptolemy worked out about 2000 years ago,
see Figure 3.
PostGIS can also emit ESRI shapefiles (map files) which can be viewed via
ESRI’s free map viewers, ArcExplorer or ArcGIS Explorer – so that’s another
way to make star maps.
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Figure 3: PostGIS stereographic projections via OpenJUMP
spot the
Plough
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5 The Solar System
At the second session Andrew began to discuss the solar system.
Figure 4: Solar system montage [11]
Figure 4 shows a nice photo montage released by NASA with images
acquired by various spacecraft, and captioned:
This is a montage of planetary images taken by spacecraft managed by the Jet
Propulsion Laboratory in Pasadena, CA. Included are (from top to bottom)
images of Mercury, Venus, Earth (and Moon), Mars,
Jupiter, Saturn, Uranus and Neptune.
…
Pluto is not shown as it is not a planet and
no spacecraft has yet visited it.
The inner planets (Mercury, Venus,
Earth, Moon, and Mars) are roughly
to scale to each other; the outer planets
(Jupiter, Saturn, Uranus, and Neptune) are roughly to scale to each other.
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5.1 The Sun
The sun (aka Sol), our star, is a massive ball of hot gas comprising about 75%
hydrogen and 25% helium, which is typical of stars. It must not be viewed
directly by telescope at risk of very serious injury.
Figure 5 shows a photograph taken
with a 4" Maksutov telescope and
Figure 5: Sun with sunspots [12]
foil-filter ND 4.
Sunspots were first spotted (by
Gallileo?) in 1611 and their
occurrence has been found to
follow an 11 year cycle [13],
which indicates solar activity.
More sunspots mean increased solar activity, when great blooms of radiation
known as solar flares or bursts of solar material known as coronal mass ejections
(CMEs) shoot off the sun's surface. The highest number of sun spots in any given
cycle is designated "solar maximum," while the lowest number is designated
"solar minimum." Each cycle varies dramatically in intensity with some solar
maxima being so low as to be almost indistinguishable from the preceding
minimum.
Sunspots are regions where the sun’s surface is cooler because magnetic field
fluctuations prevent material coming to the surface. They appear in pairs and
groups. The number of spots waxes and wanes during the 11 year solar cycle.
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The cycle is caused by the dynamic nature of the sun’s magnetic material, leading
to a magnetic pole reversal (flip) every 11 years – approximately.
The solar cycle also affects the emission of short-wavelength solar radiation,
including UV and X-rays, which are emitted by very hot gases at temperatures >
millions °C. A letter in Nature[14] suggests that low solar activity, as observed
during recent years, drives cold winters in northern Europe and the United
States.
UV radiation increases the temperature and density of the Earth’s atmosphere and
The large variations imply that satellites will decay more rapidly during periods
of solar maxima and much more slowly during solar minima [15].
5.2 The Planets [16]
Pluto was relegated by the International Astronomical Union from its position as
the Earth’s ninth planet to a new category, dwarf planet, in 2006. Pluto, Eris and
the asteroid Ceres became the first dwarf planets. Unlike planets, dwarf planets
lack the gravitational muscle to sweep up or scatter objects near their orbits.[17]
The remaining eight planets divide into two groups, the inner planets: Mercury;
Venus; Earth; Mars, and the outer planets: Jupiter; Saturn; Uranus; Neptune.
Extracting from [18]:
The inner planets and outer planets are characterized by different features. The 4
inner planets are called terrestrial planets because they have a solid surface and
are similar to Earth. These planets are composed of heavy metal, such as iron
and nickel and have few or no moons. Mercury, the smallest planet, has no
moons and is comprised mostly of iron and nickel. It is one of the densest planets
in the Solar System. Venus, known for its brightness, has a rocky surface similar
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to the Moon, which is hidden by its thick yellow atmosphere. Like Mercury,
Venus has no moon. Earth is a rocky planet with a molten core, an atmosphere
that allows life to flourish, and only one moon. The last inner planet, Mars, has
two moons called Phobos and Deimos. It is a rocky planet with a red color
caused by a high concentration of iron in the rocks that comprise the surface.
The outer planets, also called Jovian planets or gas giants, are gaseous with no
solid surfaces and only liquid cores. The outer planets are so much larger than
the inner planets that they comprise 99% of the mass of the celestial objects
orbiting our Sun.
In addition Andrew provided:
•
Mercury:
closest to the Sun, looks almost lunar. Mariner 10 orbited
Mercury in and produced mosaic images [19]. Volcanic activity obscure –
Messenger [20] provided evidence of impact craters with lava flows.
•
Venus:
appears blank because it is shrouded in cloud, we don’t see the
surface. Space probes e.g. Magellan [21] can ‘see’ through the clouds. Hotter
than Mercury by virtue of its greenhouse effect – Venus reaches 700°K, 4500°C hotter than Earth. Atmospheric pressure on Venus is crushing.
•
Earth:
without the greenhouse effect [22] Earth would be a frozen
planet. Moon seems to wobble and gets larger and smaller as it approaches
and recedes in a not quite circular orbit. Moon is largest planet (wrt its
primary planet) in the solar system, having a diameter 25% that of Earth.
•
Mars:
is colder than Earth because it is much further from Sun - 10°C
at its equatorial best but mostly sub-zero. Martian atmosphere (mostly C02)
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has much lower pressure than Earth. So low that ice turns straight to vapour
– no liquid water now on Mars but some evidence of surface water in the
past. Martian surface is reddish and atmosphere is pinkish, not blue like ours.
Mars probe, Spirit has sent photos [23] and Viking has tested for microbial
presence but The biology experiment produced no evidence of life at either
landing site [24] – but there has been criticism that it might not rule out some
Antarctic like life forms [25]. Mars has two small moons Phobos (from Gr.
for fear) and Deimos (from Gr. for panic).
•
Jupiter:
big jump beyond Mars – and the intervening asteroid belt.
Asteroids are rocky, airless worlds that orbit our sun, but are too small to be
called planets. Tens of thousands of these "minor planets" are gathered in the
main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars
and Jupiter. Asteroids that pass close to Earth are called Near-Earth Objects
[16]. Jupiter is 5 times further out from the Sun than Earth and much bigger –
the Earth would fit into Jupiter’s Great Red Spot, which is a massive storm
which has raged for hundreds of years. Surface appears banded and red
spotted. Amateur astronomers may see Jupiter’s Galilean moons, or shadows
of them: Ganymede; Io; Callisto; Europa. Io is still volcanically active, not
solid throughout like our Moon. But Io is smaller than our Moon so why is it
still active? Because Io’s orbit is close to Jupiter and it feels strong tidal
(gravitational) forces which continually squeeze and relax to keep Io warm.
Volcanic activity spews Sulphur into Io’s atmosphere making it appear
reddish.
•
Saturn:
rings visible with a smallish telescope [26]. There is as a
largish gap between Saturn and its rings. They are very thin, a myriad of tiny
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particles orbiting the planet. The main gap is called the Cassini Division –
gaps are where there is no stable orbit for any particles. Saturn has 53 moons,
including Titan which is almost as big as Jupiter’s Ganymede (largest moon
in the solar system).
•
Uranus:
was discovered in 1781 by William Herschel , the first planet
discovered using a telescope. Is a gas giant i.e. no surface to stand on like
Jupiter and Saturn, aka the Jovian planets - also known (with Neptune) as an
ice giant! Has 27 moons, named from Shakespearean characters, and thin
rings – which eclipsed stars and revealed themselves. Herschel was not
allowed to name his planet Georgium Sidus (George's Star) for his royal
sponsor, George III - Uranus was the Greek god of the Heavens.
•
Neptune:
shows more ‘surface’ features (though it is a gas giant). Has
some banding, and even spots like Jupiter. Neptune is smaller and more
massive than Uranus.
All the planets except Uranus orbit the Sun in the same direction and most (except
Venus and Uranus) spin in the same direction. Uranus suffered two great impacts.
Uranus rotates east to west. Uranus' rotation axis is tilted almost parallel to its
orbital plane, so Uranus appears to be rotating on its side. This situation may be the
result of a collision with a planet-sized body early in the planet's history, which
apparently radically changed Uranus' rotation. Because of Uranus' unusual
orientation, the planet experiences extreme variations in sunlight during each 20year-long season. [27]
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Table 1 shows the principal orbit and body parameters for each planet. Andrew drew
our attention to:
•
inclination of orbit relative to the ecliptic is mainly quite slight. (Pluto is exceptional)
•
orbital eccentricity is generally low – almost circular except Mercury (and Pluto)
•
mass of Mercury is only 6% of Earth’s – mass of Jupiter is 318 x Earth’s!
•
radius of Earth is 2.6 x radius of Mercury - radius of Jupiter is 11.2 x radius of Earth
Table 1: Planetary Physical Data [28]
Mercury Venus
Earth
0.3871
0.7233
1
Sidereal period of orbit (years)
0.24
0.62
Mean Orbital Velocity (km/sec)
47.89
Orbital Eccentricity
Planetary Physical Data
Mean Distance from the Sun (AU)
Jupiter
Saturn
Uranus
Neptune
1.524
5.203
9.539
19.19
30.06
1
1.88
11.86
29.46
84.01
164.79
35.04
29.79
24.14
13.06
9.64
6.81
5.43
0.206
0.007
0.017
0.093
0.048
0.056
0.046
0.01
7
3.4
0
1.85
1.3
2.49
0.77
1.77
2439
6052
6378
3397
71490
60268
25559
25269
6357
3380
66854
54360
24973
24340
Inclination to ecliptic (degrees)
Equatorial Radius (km)
Polar Radius (km)
same
same
Mars
Mass of planet (Earth=1)
0.06
0.82
1
0.11
317.89
95.18
14.53
17.14
Mean density (grams/centimeter³ )
5.43
5.25
5.52
3.95
1.33
0.69
1.29
1.64
Body rotation period (hours)
1408
5832
23.93
24.62
9.92
10.66
17.24
16.11
Tilt of equator to orbit (degrees)
2
177.3
23.45
25.19
3.12
26.73
97.86
29.6
Number of observed satellites
0
0
1
2
30
24
8
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6 Stars
Our Sun is a pretty typical star:
•
a massive ball of hot gas
•
emits radiation (light)
•
composition by mass: 70% Hydrogen; 28% Helium; <2% other elements
•
self-powered – by nuclear fusion at core
•
surface temperature 5777 K (most stars are in the range thousands to tens of
thousands K)
•
held together by gravity
6.1 Spectrometry
In 1862 Sir William Thomson determined that heat is radiated from the sun at a rate
not more than from fifteen to forty-five times as high as that at which heat is
generated on the grate-bars of a locomotive furnace, per equal areas [29].
Thomson went on to become Lord Kelvin, after his favourite river, and to invent his
eponymous scale of temperature. Now we can use intensity v wavelength curves to
compare surface temperatures of stars. Andrew showed us how the Planck
Radiation Law predicts that a blackbody radiator will emit radiation with peak
intensity at a wavelength which depends on its surface temperature [30].
Figure 6 shows spectra for 3 different stars, including Sol. A hot blue star like Spica
(23,000K) emits radiation with peak intensity at a shorter wavelength than a cooler
red star like Antares (3,400K).
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Figure 6: Measuring the temperature of stars [31]
So we can determine the surface temperature of a star by examining its intensity v
wavelength graph. Furthermore, spectrographic analysis of absorption spectra
allows the determination of a star’s chemical composition, and possibly an
independent evaluation of it temperature, by virtue of the strength of the absorption
lines. Absorption lines can be affected by the Doppler effect and this is used to
determine the radial speed of a star – light from receding stars is redshifted and light
from approaching stars is blueshifted. Spectrometry is very useful then, but none of
this determines distance.
6.2 Measuring distance to stars
Determining the distance to stars is difficult, given the available parameters.
Apparent magnitude is always available and if we knew the absolute magnitude we
could determine distance from that. Contrariwise, it is very difficult to determine
the absolute magnitude without knowing the star’s distance.
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The most direct method for determining distance is by stellar parallax, the apparent
movement of nearby stars against the background of very remote stars caused by
viewpoint moving as the Earth orbits the Sun, see Figure 7.
Figure 7: Stellar Parallax [32]
Current technology allows us to determine the distance accurately to within a few hundred
light-years. Hipparcos mission (European Space Agency) measured the stellar parallax of
roughly 100,000 stars with precision of a few milli-arcseconds.
The stellar parallax method is good, but only for stars within about 500 parsecs (1 parsec
= 3.26 light-years) – beyond that, parallax angles are too small.
The spectroscopic parallax method is quite accurate out to about 10 kpc from Earth but it
involves ‘looking up’ the star’s luminosity, and hence absolute magnitude, from The
Hertzprung-Russell (H-R) Diagram using the star’s temperature as the key. The star’s
temperature is available by spectrometry but the H-R diagram cannot itself be derived
without luminosity data, which is often a derivative of distance. This seems rather
unsatisfactory to me.
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Given a star’s luminosity (effectively its absolute magnitude) we could determine its
distance from its apparent magnitude, so another technique for remoter stars is to seek a
standard candle star close by, whose luminosity is known independently. Cepheid variable
stars make good standard candles at distances up to 30Mpc (100Mly). Cepheids are
pulsating stars with a period directly proportional to their luminosity, more luminous
Cepheids have a longer period. Thus, the Cepheid’s luminosity can be determined by its
period, and hence its distance from Earth by its apparent magnitude. Beyond 30Mpc Type
1a Supernovae (more later – in Death of Massive Stars) have been used in a similar way as
standard candles.
Finally, a correlation has been discovered for rotating galaxies (by Tully-Fisher) between
the width of a Hydrogen line in the emission spectrum, and luminosity. More luminous
galaxies have wider emission lines. So this correlation can lead to the distance from Earth
of entire galaxies up to 100Mpc distant or more.
Figure 8: The Distance Ladder [33]
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Figure 8 shows the so-called Distance Ladder, displaying the various techniques for
determining stellar distances, and their range of efficacy. The ranges of adjacent
techniques overlap of course and astronomers use adjacent pairs to calibrate each other.
6.3 The Hertzprung-Russell (H-R) Diagram
Hertzprung and Russell independently came up with this chart, in 1911 and 1913
respectively. Effectively they plotted star temperature against absolute magnitude
and looked for a correlation. They found the Main Sequence etc and their diagram
has become indispensable in astrophysics. Figure 9 is a useful form of it because
Figure 9: The Hertzprung-Russell Diagram [34]
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all four sides are labelled with an axis. The top horizontal axis, Effective Temperature, is
equivalent to the bottom horizontal axis, Spectral Class. Annie J. Cannon devised the
Spectral Class scheme by classifying very many stars by spectral type. She labelled them
alphabetically but later they were rearranged by temperature and some classes were
dropped. The result is the astronomer’s arcane sequence Oh Be A Fine Girl Kiss Me.
Similarly, the left vertical axis, Absolute Magnitude, is effectively the same thing as the
right vertical axis, Luminosity compared to the Sun. Counter-intuitively Effective
Temperature increases from right to left and the brightest stars have negative Absolute
Magnitude, astronomers like it like that. At least Luminosity increases upwards.
90% of all stars are in the Main Sequence ranging from hot, luminous, blue stars at top left
to cool, dim, red stars at bottom right. Our Sun is a G2V star – G2 from its Spectral Class
and V from its H-R luminosity class ( V = Main Sequence ).
Stars in the upper right corner are both luminous and cool, which implies that they must be
very big because the radiation laws mean that the intensity of light per unit area radiated is
relatively low. These are the Giants and Supergiants and together they comprise about 1%
of all stars. Betelgeuse is a Supergiant, it is so big that the Earth’s orbit would easily fit
inside it – that’s my old ‘amazing fact’ about Betelgeuse.
Stars in the bottom left corner, the remaining 9%, are hot and dim, so radiation laws tell us
they must be very small. These are the White Dwarfs.
So the H-R diagram shows that all stars fall into a few distinct types. It turns out that the
types represent different stages of a star’s lifetime, from ‘birth’ to ‘death’ but my
description of that will have to wait until another day, or perhaps a few days.
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Astronomy Essay - John Coleman DACE 201111
7 References
All references are hyperlinks to web resources or the Glasgow University library
catalogue.
1. In quest of the universe
Author:
Karl F Kuhn
Publisher:
Sudbury, Mass. : Jones and Bartlett Publishers, ©1998.
2. Universe
Author:
Publisher:
Roger A Freedman; William J Kaufmann
New York : W.H. Freeman, ©2002.
3. The Heilig-Kreuz-Kirche in Bad Tölz, Germany, long time exposure.
4. Ecliptic path.jpg
5. stellarium
6. Celestia
7. Google Sky Map
8. The HYG database
9. exoplanets.csv
10. OpenJUMP
11. Solar System Montage
12. Sun_with_sunspots.JPG
13. The Sunspot Cycle
14. Solar forcing of winter climate variability in the Northern Hemisphere
15. ORBITAL MECHANICS - Perturbations from Atmospheric Drag
16. Solar System Exploration - Planets
17. Solar System Exploration - Dwarf Planets
18. Inner and outer planets
19. Mercury - Mariner 10 Global mosaic of Mercury
20. Evidence of past volcanic activity on Mercury
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Astronomy Essay - John Coleman DACE 201111
21. Magellan Mission to Venus
22. Greenhouse Gases/Effect
23. Spirit’s snapshots of Mars
24. Viking Mission to Mars
25. An exobiological strategy for Mars exploration - The search for extant life
26. Solar System Exploration - Planets/Saturn/Rings
27. Solar System Exploration - Planets/Uranus
28. Planetary Physical Data
29. On the Age of the Sun’s Heat
30. Radiation Laws
31. Measuring the Temperature of Stars - Slide 7
32. Determination of Distance - Slide 10
33. Cosmic Distance Ladder – Slide 3
34. The Hertzprung-Russell Diagram
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