Star Basics
Today we’re going to be talking about stars. Stars are one of the most beautiful objects in our night sky, shining brightly to help sailors navigate the ocean or to create intricate constellations, some of which honestly don’t make any sense. How someone looks at a line of stars in the sky and says “yup, that’s an elephant” is beyond my understanding, but let’s move on.
In addition to their beauty, Stars are the most widely recognized astronomical objects, and represent the most fundamental building blocks of galaxies. The age, distribution, and composition of the stars in a galaxy trace the history, dynamics, and evolution of that galaxy. Hence, by studying the stars in say our own Milky Way or our neighboring galaxy, we can understand how these humungous formations were created in the first please. Additionally, stars are responsible for the manufacture and distribution of heavy elements such as carbon, nitrogen, and oxygen, and their characteristics are intimately tied to the characteristics of the planetary systems that may form about them. We see this clearly in our own solar system, where the elements in the sun tell us about the available elements left over for the rest of the planets. Therefore, the study of the birth, life, and death of stars is central to the field of astronomy.
The definition of a star is fairly straightforward; a star is any object that is sufficiently massive that it can ignite the fusion of elements in its core due to the gravitational pressures inside the object itself. This is hence why some stars are 10 times bigger than others and why stars go through extremely different lifecycles.
Stars are formed from the clouds of interstellar dust and scattered throughout most galaxies, officially called nebulae. A familiar example of such as a dust cloud is the Orion Nebula. You may remember one of the first episodes of this podcast in which I talked about our sun and how it too was born out of such a nebula. These nebulae are relatively common in our galaxy and other similar galaxies. A typical nebula is many light-years across and contains enough mass to make several thousand stars the size of our sun. The majority of the gas in nebulae consists of molecules of hydrogen and helium — but most nebulae also contain atoms of other elements, as well as some surprisingly complex organic molecules. These heavier atoms are remnants of older stars, which have exploded in an event we call a supernova. The source of the organic molecules is still a mystery.
Because these nebulae were formed by exploding stars, there are some irregularities in their mass distribution. Exploding stars aren’t the most precise in how they decide to fling out their matter, and so the areas of this nebula that are slightly denser than the rest slowly pull the gas molecules around them closer and closer together, causing the cloud to collapse. Some astronomers think that a gravitational or magnetic disturbance causes more of these dense spots to occur which hence encourages the nebula to collapse even more. As the gases collect, they lose potential energy, which results in an increase in temperature.
As the collapse continues, the temperature increases. The collapsing cloud then separates into many smaller clouds, each of which may eventually become a star. The core of the cloud collapses faster than the outer parts since it is the heaviest part, and the cloud begins to rotate faster and faster to conserve angular momentum. When the core reaches a temperature of about 3,100 degrees Fahrenheit, the molecules of hydrogen gas break apart into hydrogen atoms. Eventually the core reaches a temperature of 17,500 degrees Fahrenheit, and it begins to look like a star when fusion reactions begin. When it has collapsed to about 30 times the size of our sun, it becomes a protostar.
Protostars aren’t hot enough for the fusion that defines a star to start taking place, which is hence why they are called protostars and not just stars. Protostars are usually surrounded by dust, which blocks the light that they emit, so they are difficult to observe in the visible spectrum. They are also generally surrounded by disks of material, which will later form planets, comets, asteroids, or other space objects.
The protostar also rotates. As the it does so, it generates a strong magnetic field. The magnetic field also generates a strong protostellar wind, which is an outward flow of particles into space. Many protostars also send out high-speed streams or jets of gas into space. Usually there are two jets flowing out along the rotation axis of the protostar. Eventually the wind and the jets clear away the extra gas around the protostar and allow the protostar to come into view.
When the pressure and temperature in the core become great enough to sustain nuclear fusion, at about 18 million degrees Fahrenheit, the outward pressure generated by the protostar acts against the gravitational force compressing it. At this stage the core is about the size of our sun. The remaining dust envelope surrounding the star heats up and glows brightly in the infrared part of the spectrum. At this point the visible light from the new star cannot penetrate the envelope. Eventually, radiation pressure from the star blows away the envelope and the new star begins its evolution. The properties and lifetime of the new star depend on the amount of gas that remains trapped. A star like our sun has a lifetime of about 10 billion years and is just middle-aged, with another five billion years or so left.
A star the size of our Sun requires about 50 million years to mature from the beginning of the collapse to adulthood. Stars are fueled by the nuclear fusion of hydrogen to form helium deep in their interiors. The outflow of energy from the central regions of the star provides the pressure necessary to keep the star from collapsing under its own weight, and the energy by which it shines. The official name of this process is “stellar nucleosynthesis,” and it is the source of many of the elements in the universe heavier than hydrogen and helium. So, from stars like the Sun, the future universe will get such elements as carbon, which it will make as it ages. Very “heavy” elements, such as gold or iron, are made in more massive stars when they die, or even the catastrophic collisions of neutron stars.
How does a star do this “stellar nucleosynthesis” and not blow itself apart in the process? The answer is something called hydrostatic equilibrium. While that is quite a fancy terms, it basically means that the gravity of the star’s mass, which pulls the gases inward, is balanced by the outward pressure of the heat and light — the radiation pressure — created by the nuclear fusion taking place in the core.
Although our solar system only has one star, most stars like our sun are not solitary, but are binaries where two stars orbit each other, or multiples involving even more stars. In fact, just one-third of stars like our sun are single, while two-thirds are multiples — for instance, the closest neighbor to our solar system, Proxima Centauri, is part of a multiple system that also includes Alpha Centauri A and Alpha Centauri B.
Now that we know how stars work, let’s look a little deeper at the different characteristics that are used to describe them. First up is brightness, which is described in terms of magnitude and velocity. The magnitude of a star is based on a scale more than 2,000 years old, devised by Greek astronomer Hipparchus around 125 BC. He numbered groups of stars based on their brightness as seen from Earth — the brightest ones were called first magnitude stars, the next brightest were second magnitude, and so on up to sixth magnitude, the faintest visible ones. Quite a straightforward categorization if I do say so myself. Of course, astronomers in the future had to complicate things just a little bit. Nowadays, astronomers refer to a star’s brightness as viewed from Earth as its apparent magnitude, but since the distance between Earth and the star can affect the light one sees from it, they now also describe the actual brightness of a star using the term absolute magnitude, which is defined by what its apparent magnitude would be if it were 10 parsecs or 32.6 light years from Earth. The magnitude scale now runs to more than six and less than one, even descending into negative numbers — the brightest star in the night sky is Sirius, with an apparent magnitude of -1.46.
Luminosity is the power of a star — the rate at which it emits energy. Although power is generally measured in watts, which is the case for the sun which has a luminosity of 400 trillion watts, the luminosity of a star is usually measured in terms of the luminosity of the sun itself. For example, Alpha Centauri A is about 1.3 times as luminous as the sun. To figure out luminosity from absolute magnitude, one must calculate that a difference of five on the absolute magnitude scale is equivalent to a factor of 100 on the luminosity scale. For instance, a star with an absolute magnitude of 1 is 100 times as luminous as a star with an absolute magnitude of 6. Overall, the brightness of a star depends on its surface temperature and size.
Next up is color. When you look up at the night sky, all the stars appear to be bright white flecks of light. But this isn’t the full truth. Stars come in a range of colors, from reddish to yellowish to blue. The color of a star depends on surface temperature. A star might appear to have a single color, but actually emits a broad spectrum of colors, potentially including everything from radio waves and infrared rays to ultraviolet beams and gamma rays. Different elements or compounds absorb and emit different colors or wavelengths of light, and by studying a star’s spectrum, one can divine what its composition might be.
After that is surface temperature. Astronomers measure star temperatures in a unit known as the kelvin, with a temperature of zero K, which is also called “absolute zero,” 459.67 degrees Fahrenheit. A dark red star has a surface temperature of about 2,500 K or 4,040 Fahrenheit, a bright red star has a temperature around 3,500 K or 5,840 Fahrenheit; the sun and other yellow stars clock in at about 5,500 K or 9,440 Fahrenheit; and finally, a blue star is about 10,000 K or 17,540 Fahrenheit to 50,000 K or 89,540 Fahrenheit.
The surface temperature of a star depends in part on its mass and affects its brightness and color. Specifically, the luminosity of a star is proportional to temperature to the fourth power. For instance, if two stars are the same size but one is twice as hot as the other in kelvin, the former would be 16 times as luminous as the latter.
The next characteristic of a star is its size. Astronomers generally measure the size of stars in terms of the radius of our sun. For instance, Alpha Centauri A has a radius of 1.05 solar radii. Stars range in size from neutron stars, which can be only 12 miles wide, to supergiants roughly 1,000 times the diameter of the sun.
The size of a star affects its brightness. Specifically, luminosity is proportional to radius squared. For instance, if two stars had the same temperature, if one star was twice as wide as the other one, the former would be four times as bright as the latter.
Following that is the characteristic of mass. Astronomers represent the mass of a star in terms of the solar mass, the mass of our sun. As you can see, there’s a pattern emerging here. The reason scientists use the solar unit for so many of these characteristics is because it is easier to compare other stars to our sun and express their characteristics in terms of our sun. Also, while we can get mor exact readings for the characteristics of our sun, we cannot determine those characteristics as accurately for other stars, and hence it is smarter to compare them to the sun, as is the case for solar mass. For instance, Alpha Centauri A is 1.08 solar masses.
Stars with similar masses might not be similar in size because they have different densities. For instance, Sirius B is roughly the same mass as the sun, but is 90,000 times as dense, and so is only a fiftieth its diameter. The mass of a star affects its surface temperature.
Then we have magnetic field, which is quite an interesting characteristic. Stars are spinning balls of roiling, electrically charged gas, and thus typically generate magnetic fields. When it comes to the sun, researchers have discovered its magnetic field can become highly concentrated in small areas, creating features ranging from sunspots to spectacular eruptions known as flares and coronal mass ejections. Hence it is difficult for researchers to accurately describe a faraway star’s magnetic field since they don’t know how sunspots, or other such magnetic features apply to that star. However, scientists do know that the average stellar magnetic field increases with the star’s rate of rotation and decreases as the star ages.
The last characteristic is metallicity. The metallicity of a star measures the proportion of “metals” it has, which basically means any element heavier than helium. Three generations of stars can be determined based on metallicity, Population III, Population II and Population I, each with increasing levels of metallicity. Astronomers have not yet discovered any of what should be the oldest generation of star, which are the Population III stars that were born in a universe without “metals.” When these stars died, they released heavy elements into the cosmos, which Population II stars incorporated relatively small amounts of. When a number of these died, they released more heavy elements, and the youngest Population I stars like our sun contain the largest amounts of heavy elements.
Now that we know all about the different features of stars, lets look at how they are categorized. Stars are typically classified by their spectrum in what is known as the Morgan-Keenan or MK system. There are eight spectral classes, each analogous to a range of surface temperatures — from the hottest to the coldest, these are O, B, A, F, G, K, M and L. Each spectral class also consists of 10 spectral types, ranging from the number 0 for the hottest to the numeral 9 for the coldest.
Stars are also classified by their luminosity under the Morgan-Keenan system. The largest and brightest classes of stars have the lowest numbers, given in a sort of twisted Roman numeral system where Ia is a bright supergiant, Ib is a regular supergiant, II is a bright giant, III is a giant, IV is a subgiant, and V is a main sequence or dwarf. A complete MK designation includes both spectral type and luminosity class — for instance, the sun is a G2V.
There is also another way to categorize stars using something called the Hertzsprung-Russell Russell diagram, which is a plot of the temperature of a star and its brightness. If you take a whole bunch of stars and plot their temperature and their brightness, with one point for each star on the diagram, you find something surprising. It turns out that stars don’t have all sorts of color and brightness combinations. Instead, there is a stripe running diagonally that the vast majority of stars live on. This stripe runs from the dim, red end to the bright, blue end.
This stripe is known as the main sequence, and stars that burn hydrogen in their cores, which is the primary fuel source for the vast majority of a star’s life, will live somewhere on this stripe. As stars age, they slowly and gently move up the track along the main sequence, becoming steadily brighter and bluer as the eons go by.
How long they live on that track, steadily burning hydrogen in their cores, depends on how massive they are. A low-mass red dwarf can spend trillions of years on the main sequence, while a giant star bigger than our sun may only last a few million years at best. Once hydrogen fusion ends inside of the core of a star, it moves off the main sequence and evolves in different directions. Large stars become red giants, which occupy their own positions on the Hertzsprung-Russell diagram. Other stars might zigzag back and forth, alternating between blueness and redness as heavy elements attempt to fuse deep in their hearts.
Armed with the Hertzsprung-Russell diagram, we can see what truly defines a star: it’s an object that lives on the main sequence of that diagram. It’s an object that burns hydrogen and steadily evolves along that narrow strip connecting its brightness to its temperature. Things that exist outside that strip are either giants attempting to fuse heavier elements in a futile attempt to stay burning, or dead and decaying remnants like white dwarfs and neutron stars.
You may also be curious about what the structure of a general star is like. The structure of a star can often be thought of as a series of thin nested shells, somewhat like an onion. A star during most of its life is a main-sequence star, which consists of a core, radiative and convective zones, a photosphere, a chromosphere and a corona. The core is where all the nuclear fusion takes places to power a star and is also the hottest part of a star. In the radiative zone, energy from these reactions is transported outward by radiation, like heat from a light bulb, while in the convective zone, energy is transported by the roiling hot gases, like hot air from a hairdryer. This energy transfer can take and extremely long time, for our own sun it takes 170,000 years for energy to exit the core and then reach the top of the convective zone.
Massive stars that are more than several times the mass of the sun are convective in their cores and radiative in their outer layers, while stars comparable to the sun or less in mass are radiative in their cores and convective in their outer layers. Intermediate-mass stars of spectral type A may be radiative throughout. The convective zone is quite a bit cooler than the radiative zone, which is another interesting difference to take note of.
After those zones comes the part of the star that radiates visible light, the photosphere, which is often referred to as the surface of the star. The temperature of this layer can vary from star to star; however, it is extremely hot, hot enough to actually melt diamonds. After that is the chromosphere, a layer that looks reddish because of all the hydrogen found there. This is where things such as solar flares and sunspots occur. Finally, the outermost part of a star’s atmosphere is the corona, which if super-hot might be linked with convection in the outer layers.
Now that we’ve talked about how stars are made and all of their features, you may be curious as to how stars actually die. Well, the death of a star can vary depending on its mass. In general, the larger a star, the shorter its life, although all but the most massive stars live for billions of years. When a star has fused all the hydrogen in its core, nuclear reactions cease. Deprived of the energy production needed to support it, the core begins to collapse into itself and becomes much hotter. Hydrogen is still available outside the core, so hydrogen fusion continues in a shell surrounding the core. The increasingly hot core also pushes the outer layers of the star outward, causing them to expand and cool, transforming the star into a red giant.
If the star is sufficiently massive, the collapsing core may become hot enough to support more exotic nuclear reactions that consume helium and produce a variety of heavier elements up to iron. However, such reactions offer only a temporary reprieve. Gradually, the star’s internal nuclear fires become increasingly unstable — sometimes burning furiously, other times dying down. These variations cause the star to pulsate and throw off its outer layers, enshrouding itself in a cocoon of gas and dust. What happens next depends on the size of the core.
For most stars, such as our own sun, the process of ejecting its outer layers continues until the stellar core is exposed. This dead, but still ferociously hot stellar cinder is called a White Dwarf. White dwarfs, which are roughly the size of our Earth despite containing the mass of a star. If the white dwarf forms in a binary or multiple star system, it may experience a more eventful demise as a nova. If a white dwarf is close enough to a companion star, its gravity may drag matter — mostly hydrogen — from the outer layers of that star onto itself, building up its surface layer. When enough hydrogen has accumulated on the surface, a burst of nuclear fusion occurs, causing the white dwarf to brighten substantially and expel the remaining material. Within a few days, the glow subsides and the cycle starts again
Main sequence stars over eight solar masses are destined to die in a titanic explosion called a supernova. A supernova is not merely a bigger nova. In a nova, only the star’s surface explodes. In a supernova, the star’s core collapses and then explodes. The best example of a supernova is the Crab Nebula, in Taurus. The core of the original star is left behind as the rest of its material is blasted to space. Eventually, the core could compress to become a neutron star or a black hole.
And that’s all for the basics of how stars work. I hope you enjoyed learning about all of their cool features, such as how they are classified and how they are made. Although I touched upon the death of stars in this episode, there’s still a lot to delve into about the ends of stars so next time I’ll be talking about the magnificent deaths of stars. Its quite an interesting topic, so make sure to tune on in!
