STARS - Student Notes

On a clear, dark night, a few thousand stars are visible to the naked eye. Many more become visible through binoculars, and with a powerful telescope we can see so many stars that we could never hope to count them. Like individual people, each individual star is unique. Like the human family, all stars share much in common.

Today we know that stars are born from clouds of interstellar gas, shine brilliantly by nuclear fusion for millions or billions of years, and then die, sometimes in dramatic ways. This chapter outlines how we study and categorize stars and how we have come to realize that stars, like people, change over their lifetime.

SNAPSHOT OF THE HEAVENS

We see only a brief moment in any star’s life, and our collective snapshot of the heavens consists of frozen moments for billions of stars.

What do we now know about stars?

All stars form in great clouds of gas and dust They begin their life with roughly the same chemical composition Star’s mass at birth: about ¾ hydrogen and ¼ helium; less than 2% heavier elements During most of star’s life, rate at which it generates energy depends on balance between inward pull of gravity and outward push of internal pressure from nuclear fusion in core

Why are the stars we see different from each other?

Differ in mass We see different stars at different stages of their lives

Early classification of stars: brightness and location in sky

Greek letters: Alpha is brightest in constellation. Alpha Leo, a.k.a. Regulus

Why is a star bright?

Extremely powerful OR unusually close

Are stars that appear next to each other in the sky necessarily neighbors?

They could lie at significant differences from the Earth

15.2 STELLAR LUMINOSITY

Luminosity of a star – the total amt.of power it radiates into space (expressed in watts)

The Sun’s luminosity is 3.8 x 10 (26) watts

A star’s luminosity cannot be measured directly.

Sun and Alpha Centauri A have the same luminosity, but the Sun is much, much brighter. Alpha Centauri A is about 270,000 times farther from the earth than the Sun

Apparent brightness – The amount of light that actually reaches us. Also called “flux.”

Obeys the inverse-square law – ex. If we viewed the Sun at twice the distance it would appear 2(2) or 4 times dimmer.

Luminosities of stars are compared to the Sun’s luminosity (L-Sun = 3.8 x 10 (26) watts)

Proxima Centauri is 0.0006 L-Sun; Betelgeuse is 38,000 L-Sun

A star’s apparent brightness is measured with a detector, such as a CCD, that records how much energy strikes its light-sensitive surface each second.

Total luminosity and total apparent brightness take into account all photons across the entire electromagnetic spectrum.

Once a star’s apparent brightness has been measured, the next step in determining it’s luminosity is to measure its distance. The most direct way to measure the distances to stars is with stellar parallax. This is the small shift in a star’s apparent position caused by the Earth’s motion around the Sun. Astronomers measure stellar parallax by comparing observations of a nearby star made 6 months apart. The nearby star appears to shift against the background of more distant stars because we are observing it from 2 opposite points of the earth’s orbit. Parallax can only be used to measure the distances to stars within a few hundred light years away. (our “local” solar neighborhood)

Proxima Centauri, the closest star to us, has a parallax angle of only 0.77 arcsecond

The distance to an object with a parallax angle of 1 arcsecond is one parsec (pc)

1 pc = 3.26 light-years = 3.09 x 10(13) km

Enough stars have measurable parallax to give us a fairly good example of the many different types of stars. 300 stars within 33 light-years (10 parsecs) of the Sun.

REVIEW: (Ch.2, p.44-45) angular distance.

Width of stretched out hand ~ 20 degrees Width of fist ~ 10 degrees Finger width ~ 1 degree 60 arcminutes per degree; 60 arcseconds per arcminute

Apparent magnitude – a scale that compares how bright different stars appear in the sky.

The lower the number, the higher the absolute magnitude. Sirius, the brightest star, has an absolute magnitude of –1.46 (look at table F2 in appendix)

Absolute solar magnitude – the apparent magnitude a star would have if it were at a distance of 10pc from the Earth. If the Sun were 10pc from the Earth, it would have an absolute magnitude of about 4.8. Visible, but not conspicuous on a dark night.

15.3 STELLAR SURFACE TEMPERATURE Surface temperature is determined directly by the star’s color or spectrum.

A stars surface temperature determines the color of light it emits.

The naked eye can distinguish colors only for the brightest stars. Colors of stars become more evident when viewed through binoculars or telescope.

Betelgeuse, a cool, red star would look much brighter through a red filter than a blue filter.

SPECTRAL TYPE

Emission and absorption lines in star’s spectrum provide accurate way to measure surface temperature. Stars displaying spectral lines of highly ionized elements must be fairly hot; those displaying spectral lines of molecules must be relatively cool.

Astronomers classify stars according to surface temperature by assigning a spectral type determined from spectral lines present in the star’s spectrum. O-B-A-F-G-K-M

The hottest stars are type O. (challenge: come up with a good pneumonic device)

Each spectral type is subdivided into numbered subcategories. The Sun is G2.

History of the Spectral Sequence – read pp. 477-479

Henry Draper, Edward Pickering and his team of “female computers”: Williamina Fleming, Annie Jump Cannon, Cecilia Payne-Gaposchkin.

STELLAR MASSES

Most important property of a star. Most dependable method of “weighing” a star is to use Newton’s version of Kepler’s Third Law. REVIEW: Universal law of Gravitation (p.140) Stellar masses can only be measured in binary star systems in which the orbital properties of the two stars have been determined.

Visual binary – a pair of stars that we can see distinctly as the stars orbit each other.

Example: Mizar (second star in the handle of the Big Dipper) (p.480, fig. 15.5)

Eclipsing binary – a pair of stars that orbit in a plane of our line of sight. When neither star is eclipsed (or blocked), we see the combined light of both stars. When one star eclipses the other, the apparent brightness of the system drops. Example: Algol (the “demon” star in the constellation Perseus)

If binary system is neither visual nor eclipsing, we may be able to detect its binary nature by observing Doppler shifts in its spectral lines. (p.481, fig.15.8)

THE HERTZSPRUNG-RUSSELL DIAGRAM

Most stars fall along the main sequence – upper left to lower right. These stars fuse hydrogen into helium in their cores and have a wide range of life spans, which depend on their mass. Higher mass stars on main sequence have shorter life spans. Supergiants are very large in addition to being very bright. Giants are somewhat smaller in radius and lower in luminosity, but still much brighter than main sequence stars of same spectral type. The hot, white, small radius stars near the lower left are called white dwarfs.

A star has a limited supply of core hydrogen and therefore can remain as a hydrogen-fusing main sequence star for a limited time - the star’s main sequence lifetime.

Our Sun’s main sequence lifetime is about 10 billion years. A 30 x M-Sun star has 30 times more H than the Sun, but burns it with a luminosity that is 30,000 times greater. It’s lifetime is 30/30,000 = 1/10,000 as long as the Sun – corresponding to a lifetime of only a few million years. This is a very short time, cosmically speaking. This is one reason why massive stars are so rare. Most of the massive stars that have ever been born are long since dead.

Giants and Supergiants are stars nearing the ends of their lives because they have already exhausted their core hydrogen. Surprisingly, they grow more luminous when they begin to run out of fuel (next topic outline) If a supergiant were in the same spot as the Sun, it would engulf all of the planets through Jupiter!

PULSATING VARIABLE STARS

Not all stars shine steadily like our Sun. Any star that significantly varies in brightness with time is called a variable star. In a futile quest for steady equlibrium, the atmosphere of a pulsating variable star alternately expands and contracts. The periods of these stars can range from several hours to several years! Most pulsating variable stars occupy a strip (called the instability strip) on the H-R diagram that lies between the main sequence stars and the red giants (p.487, fig.15.12) The most luminous pulsating variable stars are called Cepheid variables or Cepheids. (Named after the first discovered star of this category, Delta Cephei) Henrietta Leavitt, a Harvard astronomer, discovered, in 1912, that the periods of these stars are very closely related to their luminosities. The most well- known Cepheid star is Polaris, the North Star.

STAR CLUSTERS

All stars are born from giant clouds of gas. Because a single interstellar cloud can contain enough material to form many stars, stars almost inevitably form in groups. Many stars still congregate in the groups in which they formed.

The two basic types of groups: open clusters and globular clusters.

Open clusters are always found in the disk of a galaxy and can contain up to several thousand stars and typically span about 30 l.y. (10pc) The most famous open cluster is is the Pleiades (also called “the seven sisters”) in the constellation Taurus.

Globular clusters are found in both the halo and disk of our galaxy. They can contain more than a million stars, concentrated in a ball typically 60-150 l.y. across. The innermost region of a globular cluster can have 10,000 stars packed within just a few light years!

What would the view be like from a planet in the midst of a globular cluster?

How do globular clusters gradually lose stars and become more compact?

Star clusters are useful to astronomers for 2 key reasons: All the stars in a cluster lie within the same distance from Earth All the stars in a cluster formed at about the same time (within a few million years of each other)