Star

Stars are extremely large, luminous bodies of gas. They are the most obvious features found in the universe. They are principally composed of hydrogen that is undergoing nuclear fusion to become helium. Our Sun is the nearest star to Earth, at a distance averaging 93 million miles. The Earth orbits the Sun in a period of approximately 365.25 days, and this defines the year. The diameter of the Sun, which is a typical star, is about 870,000 miles and its power output is about 10²⁶ watts. The temperature inside the Sun is estimated to be in excess of ten million degrees, and this is hot enough for nuclear reactions to occur.

Measuring stellar distances

The oldest method of measuring the distance from our solar system to a distant star is the parallax method. To use this method, astronomers measure the right ascension on the sky of the star at two times of the year, half a year apart. The two measurements will differ by a small angle with respect to the most distant stars in that region of the sky. Exactly half this angle is the parallax angle, having symbol p. This is the angle that the star makes with the sun and the position of the earth at a right angle with that star. The distance s of the star, in astronomical units (AU), is:

In the range of the very small angles typically encountered, the cotangent of the angle measure (in radians) is very nearly equal to the reciprocal, and thus:

where p is measured in seconds of arc.

The cotangent of one second (1/3600 of a degree) of arc is approximately 206,264.81. No parallax angle for any star will be larger than one second. Therefore astronomers initially defined a unit of stellar distance, the parsec (symbol pc), from this relationship. One parsec is the distance corresponding to a parallax angle of one second of arc. Hence:

However, the error of measurement of parallax angle is 0.005 arc seconds, and beyond a distance of 100 parsecs, this error becomes significant. 700 stars are near enough to measure their distances directly by using parallax. To measure distances further out than this, astronomers typically use absolute and relative magnitudes, or they apply Hubble's Law to the star's estimated redshift.

Stellar positions and movements

The most common system for describing the position of a star in the sky is the equatorial system. This system uses two coordinates:

1. Right ascension on the sky, or the number of hours required for the earth to rotate before an observer can see the star at its highest point in the sky. The zero for right ascension is midnight on the day of the vernal equinox.
2. Declination, or the north-south angle between the star and the celestial equator.

All stars move, but the most distant stars are considered "fixed" because their motion would be undetectable. The proper motion (symbol m) of any star is the angular velocity of its position across the sky. This describes the motion at right angles to the line of sight of the observer. To convert this to actual tangential velocity, multiply the tangent of this angular velocity by the star's distance.

The motion in line of sight, or radial velocity, is currently determined from spectral shift.

Measuring stellar magnitudes

The visual magnitude system is defined as follows: a star of any given magnitude is about 2.512 times as bright as is a star of the next magnitude. Hipparchus devised the magnitude system, and Ptolemy refined it further. By convention, an arbitrary sample of the twenty brightest stars that they could observe were assigned to the first magnitude, and the stars that they could barely observe were assigned to the sixth. Sixth-magnitude stars are actually 100 times less bright than first-magnitude stars. Magnitude levels between these extremes are assigned on a logarithmic scale. Thus, given two stars of brightness l₁ and l₂, their magnitude difference (V₂ - V₁) relates to their respective brightnesses in this way:

The absolute magnitude of any star is the visual magnitude that it would have if it were ten parsecs distant. To convert apparent magnitude V to actual magnitude M, use this formula:

where s₀ is the standard distance. This distance is ten parsecs, or about 2,062,650 AU.

Brightness declines with the square of distance, and squares correspond to doubling of logarithms. One must then multiply that result by 2.5 to stay within the magnitude scale.

Stellar colors and spectra

The color of a star is objectively quantifiable. To determine color, astronomers view the star through a variety of colored filters and compute color indices as the differences in apparent magnitudes through the various filters. Stellar colors vary, in order from the coolest to the hottest, from red to yellow to white to blue-white to blue or violet. This is the same gamut of colors that a black body shows as its temperature rises.

In addition, each star has a unique spectrum, which depends on the gases and other elements that it contains, and their distribution. A spectrum can serve two purposes:

1. It can serve as a unique signature for the star, to distinguish it from other stars.
2. It can provide information on the star's radial velocity vis-à-vis the earth.

To accomplish the latter, astronomers note the placement of various lines in the spectrum and then determine the star's likely constituent elements from the spacing of those lines. Lines that are out of place are shifted, either toward the blue or toward the red. Nearly all stellar spectra are shifted toward the red; this redshift indicates a recession, either of the star or of the part of space where the star resides.

Spectral types

Hertzsprung-Russell Diagram

In the late nineteenth century, astronomers at the Harvard University observatory developed the first classification scheme for stellar spectra that would become known as the Harvard spectral classification. In 1924, Annie Jump Cannon refined the classification from the original A-Q gamut to the familiar "OBAFGKM" gamut. Astronomers have since added classes to this range at the high end and the low.

The classic Harvard spectral classes are O, B, A, F, G, K, and M. Each of these has ten subclasses, varying from 0 to 9 in order of decreasing stellar temperature. Thus, for example, the next class after an F9 star is a G0 star. Recently astronomers recognized one class of stars hotter than the O stars (the very hot Wolf-Rayet stars) and three classes of stars (the N, R, and S stars) cooler than the M stars. (Some astronomers include the N and R stars in one class, the C stars, for the carbon compounds that their spectra exhibit.)

In addition to the spectral type, astronomers today add a luminosity class, which varies from I to VI in order of decreasing brightness. The sun's spectral type is G2 and its luminosity class is V (five).

Class	Temperature	Color	Elements	Notes
W	106,000 K	Violet	Ionized helium, carbon, oxygen, nitrogen	Wolf-Rayet stars. Additional subclasses include WC (overabundant carbon and oxygen) and WN (overabundant nitrogen)
O	30,000 K	Blue	Ionized Helium, nitrogen, oxygen	Weak Balmer lines (hydrogen) at higher subclasses.
B	13,000 K to 20,000 K	Blue	Neutral helium; ionized silicon, oxygen and magnesium.	Hydrogen (Balmer lines) appear in strength
A	75,00 to 10,000 K	Blue-white	Hydrogen, calcium, helium	Balmer lines dominant. K lines (calcium) now appearing.
F	7,000K to 9,000K	White-yellow	Hydrogen, calcium, iron, manganese, sodium	Balmer lines weakening. K lines stronger.
G	5,200 to 6,000K	Yellow	Calcium, hydrogen, other metals	Balmer lines weaker still. K lines dominant. Metals now appearing.
K	4000K to 5100K	Orange	Calcium, neutral metals, titanium oxide
M	3000K	Red	Titanium oxide, iron iodide	Strong molecular bands
N,R	2300K to 2600K	Red	Carbon compounds
S	2300K to 2600K	Red	Hydrogen, zirconium oxide

In the early twentieth century, astronomers Ejnar Hertzsprung and Henry Norris Russell prepared the first plot of stellar temperature as a function of luminosity, or brightness. Other astronomers have since prepared versions of the diagram showing absolute magnitude as a function of color. This diagram shows a "main sequence" of stars for which brightness declines as temperature increases, but also shows a "white dwarf" population of very hot but dim stars, and the population of giants and supergiants that are far brighter than their temperatures would indicate.

Variable stars

Some stars vary in brightness and are known as variable stars. The star Algol in the constellation of Perseus can drop from its normal magnitude of 2.3 to magnitude 3.5. This is now known to be caused by a dim companion star orbiting Algol, which occasionally passes between Algol and the Earth, blocking some of the light. Other variable stars vary in brightness due to actual variations in the luminosity of the star itself. The time taken from one maximum brightness to the next one is called the period. The most famous of the variable stars is delta Cepheus, the first-found member of the Cepheid group of variable stars. In 1908 Henrietta Swan Leavitt noticed that the variable stars in the Magellenic Clouds (two nearby galaxies in the Local Group) had a relationship between their period and their apparent brightness. At that time galaxies outside our own (the Milky Way) had been discovered, but it was not possible to measure the distances to them. It was soon realized that the variable stars in the Magellenic Cloud were of the Cepheid type. Since Cepheid variables also occur in our galaxy it was possible measure their distances and thus convert (using the inverse square law) Leavitt's relationship between apparent brightness and period to one of actual brightness and period. Once this formula was discovered, it became possible to apply to Cepheids of unknown distance. By observing their periods, their actual brightness can be calculated and, by the inverse square law, their distance. Through observations of Cepheids in globular clusters (compact bunches of stars) in our galaxy it was shown that our galaxy is about 300,000 light-years in diameter.

Young Earth Creationism View

Young earth creationist scientists assert that materialistic explanations of the origin of stars are errant and contra-evidence and reports of stars forming are invalid. In addition, creationists cite the secular scientific literature in order to make the case that materialist explanations of star formation are inadequate:

“We don’t understand how a single star forms, yet we want to understand how 10 billion stars form.” Carlos Frenk, as quoted by Robert Irion, “Surveys Scour the Cosmic Deep,” Science, Vol. 303, 19 March 2004, p. 1750.

“Nobody really understands how star formation proceeds. It’s really remarkable.” Rogier A. Windhorst, as quoted by Corey S. Powell, “A Matter of Timing,” Scientific American, Vol. 267, October 1992, p. 30.

Old Universe View of Stars

A team of astronomers from the University of Texas at Austin McDonald Observatory recently estimated the age of one of the oldest stars in the Milky Way, HE 1523-0901, at 13.2 billion years. They used radioactive decay dating techniques. This star is close to the estimated age of the universe, 13.7 billion years.

The team leader, Dr. Anna Frebel, said that it was hard to pin down the age of a star, but that it can be inferred that chemically primitive stars must be very old, since they would have been born before many later generations of stars had provided chemical enrichment to the galaxy. A very few old stars contain huge amounts of some chemical elements, including radioactive thorium and uranium, and astronomers can use known rates of decay to deduce the ages of the stars using uniformitarian assumptions.

Frebel's team measured the uranium in HE1523-0901 using the UVES spectrograph on the Kueyen Telescope. This is one of four that together comprise the Very Larte Telescope based in Chile. She explained that although uranium had been discovered in two other stars, HE 1523-0901 also contains thorium. Uranium has a half-life of 4.5 billion years while thorium has a half-life of 14 billion years. However, HE 1523-0901 also contains other elements that can be anchored to the radioactive elements - europium, osmium and iridium. This combination provided Frebel with six 'cosmic clocks' that could be used to check each other.

The team hope to gain new experimental data and important clues about the hypothesized creation and evolution of the chemical elements shortly after the Big Bang.

Other References

The Observer's Book of Astronomy, by Patrick Moore. Published by Frederick Warne and Co. 1967.

The Cosmological Distance Ladder, by Michael Rowan-Robinson. Published by Freeman. 1985.