The Life and Death of Stars

“. . . thorough, detailed, and fascinating.”

“Perhaps the most fascinating aspect of stellar alchemy and by its implications for life on the Earth, most of the chemical elements in our bodies from the calcium in our feet to the iron that makes our blood red, were created billions of years ago in the hot interiors of long vanished stars. Therefore, we are all made of star stuff.”
—Kenneth R. Lang in The Life and Death of Stars

Of interest to readers of all ages, The Life and Death of Stars should be your “go to” popular science text for facts about the Sun, the solar system, the stars, and the Universe. It answers questions such as: How big is the Sun? How far is it from the Earth, and how do we measure its distance? How did stars come about and what keeps them hot? And what is the Universe’s ultimate fate?

The Life and Death of Stars contains stunning color photos taken by satellites and earth based observatories of supernova, nebula, clusters, and colliding galaxies. Author and professor Kenneth R. Lang provides the reader an introduction to astronomy and cosmology, explaining how the distance from the Earth to the stars and how the difference between a star’s apparent brightness and its intrinsic luminosity is determined.

The author casts a wide net. He first starts (relatively) local in space, providing information about our Sun and solar system. He then goes small, very small—to atomic and subatomic particles, the discovery of X-rays, fluorescence, uranium, radium, and radioactive decay, before looking outward to the stars.

He also artfully balances descriptive explanations with fundamental relationships—he first describes an attribute of the Sun in everyday terms, then explains what it means in terms of physics, and then generalizes and applies that knowledge to a range of stars. His selection of attributes follows the chronology of scientists’ discoveries, which are followed by short biographies of the scientists who made the discoveries.

Of the 100 billion stars in the Milky Way we can see only 5000 of them by unaided eye, and before civilization had powerful telescopes, we believed our galaxy (the Milky Way), to be the only galaxy in the Universe. Our eyes see visible light, just one band or spectrum of wavelengths of electromagnetic (EM) radiation present on Earth. Electromagnetic radiation contains wavelengths both visible and invisible – the visible spectrum provides the colors we see; the invisible spectrum includes heat, light, radio waves, X-rays, and gamma rays.

We use telescopes tuned to the wavelength of interest to observe EM waves both visible and invisible from great distance. And although EM waves differ in wavelength they all propagate at the same speed, the speed of light, and because EM radiation travels at finite speed, to look far into space is also to look backwards in time. Telescopes are in effect “time machines”.

Telescopes are no longer limited in resolution (the ability to separate objects close together) by their physical size. Using the Virtual Long Baseline Interferometry (VLBI) technique, that is, by using a number of smaller radio telescopes synchronized in time, a virtual larger radio telescope can be computationally formed. With this technique, we are just now becoming able to image the black hole at the center of the Milky way, Sagittarius A*.

The Sun is a star and what we know about the Sun is the benchmark for what we know about all stars. What is the Sun? The Sun is a hot ball of gas with no solid surface, consisting of hot atoms, mostly hydrogen, and having no net electric charge.

The Sun also has layers. We see the cooler layer of the photosphere—below that is the hotter chromosphere. The Sun’s heat and light comes from the motion of its molecules, the greater the speed of the molecule the greater the heat. Moving particles exert pressure, and the Sun’s atmosphere’s pressure extends outward. The layer above the photosphere is the corona, which can be seen on Earth during an eclipse. The corona transitions into the solar wind—gas and particles that travel outward toward the Earth and beyond.

One question about the Sun’s corona that as yet remains unanswered is how the Sun’s hot and transparent outer atmosphere, the corona can be hotter than the layer below, the photosphere. This is as puzzling to scientists as seeing water flow uphill. Scientists assume that magnetism acts as the transport mechanism pumping heat from the photosphere to the corona, but the details of how this is done are still not known.

The light spectrum emitted by the Sun provides clues for the composition of the stars. Stars have essentially the same composition of the Sun, hydrogen being the most abundant element in the Universe. But we still see different spectra coming from different stars. The spectra we see from a stars’ heated atmosphere corresponds to different conditions of temperature and pressure on hydrogen, and also spectral shift as a star moves towards or away from us (the Doppler Effect).

The core of the Sun is powered by nuclear fusion, and the energy released follows Einstein’s famous equation: e = mc^2. Every second, five million tons of matter disappears by the Sun fusing hydrogen into helium though in the past 4.6 billion years that it’s been shining, only one percent of the Sun’s mass has been used up. So you don’t need to worry about the Sun going dark just yet.

What keeps the Sun from simply blowing up? The sheer weight of the gas that surrounds the Sun’s core provides enough gravitational inward pull to match the outward pressure of its moving particles. The Sun is in hydrostatic equilibrium, that is, outward pressure and inward compression are in balance. Lucky us.

The Sun has an 11-year solar cycle as measured by increasing and ebbing solar activity. The active portion of the solar cycle includes a greater number of sunspots, solar flares and coronal mass ejections (CME).

Solar flares are very powerful, having the strength of millions of nuclear bombs exploding simultaneous though each flare’s energy release is still minor, only one one-thousandth of the energy released by the Sun every second. A coronal mass ejection is a giant magnetic bubble that rushes away from the Sun at supersonic speeds, becoming part of the solar wind that when headed towards Earth excites the Aurora Borealis, the northern lights.

We are protected on Earth from solar flares and the solar wind by Earth’s magnetic field. Weak as it is, the Earth’s magnetic field is strong enough to divert most of the extreme solar activity high above the Earth’s atmosphere. Less than 0.1% penetrates to the lower atmosphere but even that small amount can have profound influence on Earth. Solar energy entering the upper atmosphere can harm communications satellites, astronauts, and even passengers flying in aircraft near the Earth’s magnetic poles.

Stars are classified by size, from dwarf to average size stars, and giants to super giants. Stars are also classified by their color giving a rough indication of the temperature of the star’s photosphere. Bigger stars are hotter, bluer and more luminous while smaller stars are cooler and redder.

The hottest stars are so hot we cannot see them by naked eye, emitting light in the ultraviolet. The same holds true for the coolest, emitting light in the infrared. Stars can form into a variety of clusters, and those clusters can move in a variety of ways. Stars can even be runaways, ejected by galactic collisions.

The vacuum of space isn’t a true vacuum. There is material between the stars consisting interstellar gas and dust, comprised of atoms and molecules, including molecular hydrogen, ammonia, water, formaldehyde, carbon monoxide, and hydrogen cyanide. The clouds of gas can be so hot as to emit radio waves and microwave pulses, and may be so massive as to gravitationally condense to form new stars and planets.

Some day all stars will cease to shine. A star’s temperature and mass determines its lifetime and form of death. When a star similar to our Sun uses up its nuclear fuel, it will first expand and then collapse into an Earth-sized white dwarf, ejecting its outer layers into space.

A star greater than 1.46 solar masses (the Chandrasekhar limit) will first collapse into a white dwarf but will continue to collapse even further, into a neutron star or black hole. A neutron star is so dense a teaspoon of it would weigh 5 billion tons.

Where stars are close, for example a binary star consisting of a white dwarf that gravitationally pulls material from its neighbor, the pulled material can compress, heat, ignite and explode like a colossal hydrogen bomb, called a nova. The white dwarf may survive the explosion to repeat the cycle.

There are different kinds of nova depending on the amount of energy and mass ejected. For the repeating kind, the amount of light from the explosion will be the same each time. Called a “standard candle” this nova can be used as a measuring stick to measure distances from Earth to host galaxies beyond the Milky Way. There other kind of nova, the supernova occurs from the collapse of a massive star.

There’s plenty more to The Life and Death of Stars.

In the last chapter, Kenneth R. Lang shares his insight on the relationship between theory and observation and mathematics noting that the math we use today is not yet good enough to explain the very beginning of the Universe. The equations “blow up” and fail at the singularity of the “Big Bang.”

Reader friendly, filled with facts, figures, diagrams, and images taken from satellites and Earth based observatories with references to more advanced texts, The Life and Death of Stars addresses natural objects both large and small: the Sun, stars, galaxies and the Universe, but also atomic particles, subatomic particles, and electromagnetic waves. It is also illustrated in color.

The Life and Death of Stars is thorough, detailed, and fascinating.