What causes the colors seen in stars and nebulae?

Stars great and small, and their life cycles

A star’s color is critical in identifying the star, because it tells us the star’s surface temperature in the black body radiation scale. The sun has a surface temperature of 5,500 K, typical for a yellow star. Red stars are cooler than the sun, with surface temperatures of 3,500 K for a bright red star and 2,500 K for a dark red star. The hottest stars are blue, with their surface temperatures falling anywhere between 10,000 K and 50,000 K.

Stars are fuelled by the nuclear fusion reactions at their core. There is a dynamic equilibrium maintained throughout the star’s life between the expanding heat of the reactive core and gravitational forces holding the star together. Fusion produces extremely high energy. Fusion releases some of the energy that binds the particles of the nucleus together, unleashing remarkable power.

Stars begin as a mass of dust and gas dense enough to start collapsing inwards under the pressure of its own gravity. If this protostar is massive enough, it will eventually initiate a nuclear reaction in its hot, dense core. This initiates the main sequence of a star’s life cycle, when hydrogen forms helium at the star’s core through the process of nuclear fusion. Heat from the star’s core radiates outwards through the layers of the star to the photosphere, the visible surface, which emits electromagnetic energy and charged particles as a solar wind.

A star does not stay the same color throughout its lifecycle, since the surface temperature alters depending on the type of fusion reaction fuelling the star at the time. Depending on the initial mass of the star, it will evolve along the lines of one of three main star types: low-mass stars, intermediate-mass stars (like our sun) and high-mass stars.

Intermediate-mass stars are stars similar in mass to our sun. The sun is an intermediate-mass star in its main sequence, which means it is fuelled by hydrogen fusion in its core. Typically, the main sequence for an intermediate-mass star lasts around 10 billion years.

Once all the hydrogen in the core has been converted to helium by nuclear fusion, there is no energy outflow to counter the inward force of gravity and the star rapidly collapses. This in turn heats the core and the region around it to such an extent that hydrogen fusion begins in the outer layers. Even more heat is generated than in the main sequence, and the star expands to become a red giant.

When the core reaches a temperature of 100 million K, helium fusion begins. The star continues to use up hydrogen and helium until they are exhausted, which takes around 10 million years. Once all the helium in the core has been used up, the core cools again and the star undergoes a second contraction. Once more, this produces massive heat, and hydrogen fusion is initiated in the next outer layer. The star becomes a giant again, but this time a blue-hot giant. Expansion due to heat overcomes the force of gravity, and the outer layers of the star start to strip away from the star and expand out into space as a nebula.

Once the nebula fades, the core is called a white dwarf, and has a temperature of 100,000 K. This cools slowly, over billions of years, to become a black dwarf too faint to detect. This is the end of the star’s life.

High-mass stars have a mass eight or more times that of our sun. They are a thousand to a million times more luminous than the sun, and around ten times bigger in diameter. These stars are highly visible in the sky, even when they are far from the earth. They burn brighter, but their lifetimes are correspondingly much shorter than those of less massive stars.

Low-mass stars have a mass of between a tenth and a half of the sun. If their mass is below this level, they do not have sufficient gravity to sufficiently pull their material inwards to initiate nuclear fusion.

Blue stragglers have been identified as anomalously young stars in a globular cluster where the other stars are much older red giants. A globular cluster is a “swarm” of several hundred thousand stars, formed at the same time as the Milky Way galaxy. Most stars in a star cluster like this were formed about 15 billion years ago. They also spin 2 or 3 times faster than stars of a comparable size in the cluster. Pictures of globular clusters tell scientists a lot about these unusually young stars.

There are two theories for the formation of new stars within a globular cluster: they may be formed by collisions between stars, or by siphoning of material from neighboring stars “captured” by gravity as the stars pass close to each other.

By examining the light emitted by blue stragglers, astronomers have established that they have less carbon and oxygen than their neighbors. This supports the theory that the new stars form by sucking in material from their partners as they spin around each other in a binary system.

Nebulae

Nebulae form brilliantly colored spectacles, a phenomenon that becomes increasingly breathtaking as the quality of telescope and spacecraft images improves.

As we have already seen, a nebula can form in the wake of a star, either the supernova of a high-mass star, or the gas shell of an intermediate-mass star ejected when it becomes a white dwarf. The second type is known as a planetary nebula; early astronomers thought that the shells resembled the discs of planets.

Arab and Chinese astronomers first recorded the Crab nebula in 1054, when its light first reached the earth, in the constellation of Taurus. At its center are two stars, one a pulsar, a neutron star emitting regular pulses of electromagnetic radiation including radio and x-ray wavelengths.

The beautiful colors of the Orion Nebula (M42, NGC 1976) are an example of cosmic gas clouds radiating light due to excitation by radiation from nearby stars.

Invisible stars and dark matter

Many objects in the universe emit electromagnetic radiation that does not fall in the visible spectrum. We can study these objects by measuring the microwaves, x-ray and gamma radiation, and radio waves that they emit.

Quasars produce an intense electromagnetic emission ranging from x-ray to radio wave frequencies. As few other objects emit radio frequency radiation, these stand out particularly well. They alert us to the presence of black holes that form the centers of some galaxies. While a black hole has extremely high gravity and sucks in surrounding material rather than emitting radiation, the quasar surrounding the center is a measurable source of radio wave frequency.

Blazars are similar to quasars, but on an even more dramatic scale. Their emissions are the most violent phenomenon observed in the universe: a compact and highly variable energy-emitting source believed to surround super-massive and dense black holes.

Accretion discs are formed as material is passed from one star to another, for instance between two stars in a binary system, or in the active nucleus of a forming galaxy. Material is sucked towards the more massive partner and whirls around the center of its captor star in a whirlpool pattern. The hot gas accelerates and becomes hotter through friction as it falls towards the bigger star. Because it is accelerating, it emits energy in the form of infrared or x-ray radiation, which we can detect and measure from earth once it reaches us.

In addition to objects that emit radiation outside of the visible light spectrum, there is a huge proportion of matter in space that emits no radiation at all. This dark matter is known to exist because of the gravitational force it exerts, calculated using the distribution of the stars we can see and their movements. Calculations show that galaxies hold up to five times more material than we would expect. One of the ongoing lines of research in cosmology is a better understanding of this dark matter.