Humans have studied the stars for thousands of years. To many cultures, stars were the metaphor for constancy, while everything else moved and changed. Modern stellar astronomy showed that stars do change on many time scales, ranging from days to longer periods of time than human history. Stars are born, they change over their lifetimes, and they die. Along the way, they influence the chemistry and structure of their environment, and provide a home for any planets in orbit. Stellar astronomy is dedicated to studying each step of that process, treating stars both as individuals and as members of a population.
Center for Astrophysics | Harvard & Smithsonian stellar astronomers study all aspects of star birth, life, and death:
Identifying and studying protostars inside the clouds that gave them birth. These nebulas are opaque to visible light, so astronomers use submillimeter light observatories like the CfA’s Submillimeter Array (SMA) and the Atacama Large Millimeter/submillimeter Array (ALMA) to see through the gas and dust. Astronomers identified a sudden outburst from a protostar, caused by a lot of mass clumping onto it at once. Protostar Blazes Bright, Reshaping Its Stellar Nursery
And studying the magnetic field of the Sun to learn about other stars. Using data from the Sun, researchers create three-dimensional models of magnetic field generation, which can be tested against observational data from other stars. The end goal is understanding exactly how stellar magnetic fields are created, and how they influence planets in their systems. The Secret of Magnetic Cycles in Stars
Monitoring starquakes to understand the interiors of Sun-like stars. The Sun and stars like it vibrate, passing sound waves through their interiors. Just as earthquakes let geologists map Earth’s interior, these starquakes allow astronomers to measure what’s going on inside stars. Using NASA’s Kepler Observatory and other instruments designed to watch stars for long periods of time, researchers measure the fluctuations of light caused by these vibrations. Solar-Like Oscillations in Other Stars
Observing aging stars as they shed huge amounts of material into surrounding space. Old giant stars are simply too big to keep a stable grasp of their outer layers, so their surfaces pulsate and eject particles in the form of powerful winds. Using ALMA and other observatories, astronomers can identify the composition and structure of these winds. Pulsation-Driven Winds in Giant Stars
A one with 22 zeros behind it - a rough approximation of the number of stars in the observable universe.
Measuring the fluctuations in a variable star. Many old stars pulsate, but the details of the processes involved are difficult to measure in individual stars. CfA astronomers used NASA’s Hubble Space Telescope to characterize pulsations in the giant red star Betelgeuse, familiar as the “shoulder” of the constellation Orion. By measuring the speed of material in different parts of Betelgeuse’s atmosphere, they found the fluctuations are asymmetrical, much like the contractions of a human heart as it beats. Betelgeuse’s Chromosphere Beats Asymmetrically
Providing the first images of individual stars. Even with powerful telescopes, most stars are visible only as points of light. However, CfA astronomers captured an image of Betelgeuse using the Hubble, demonstrating that aging giant stars are dramatically non-spherical in shape. This study also provided the first map of the star’s surface, which showed that Betelgeuse has much larger fluctuations in temperature than our Sun. Astronomers Capture First Direct Image of A Star
The Life and Death of Stars
Stars produce nearly all the light in the sky. Stars also create the carbon, oxygen, iron, and most of the other elements planets — and life — are built from. The nuclear fusion that makes heavier elements from lighter ones is what defines a star, and the details of that fusion indicates where the star is in its life cycle.
Two things determine the unique life of a star: its mass, which is set during formation, and whether it lives its life alone or with companions. If a star is in a binary or larger association, its companions can affect its evolution through the exchange of mass or tidal forces pulling each other out of shape. Meanwhile, solitary stars like the Sun follow a set path determined by their mass.
Low-mass stars fuse hydrogen into helium at a slow rate, and live phenomenally long lives, most of them longer than the current age of the universe. Moderate-mass stars like the Sun consume the available hydrogen and helium over the course of a few billion years, then die peacefully, leaving behind a white dwarf. High-mass stars speed through their life cycle, exploding as supernovas and leaving a neutron star, black hole, or nothing at all. These explosions enrich their surroundings with new atoms and molecules, providing the materials for the next generation of stars.
Stellar astronomy studies the life cycle and structure of stars, both as individuals and as populations. By tracking the commonalities and differences that make stars what they are, we understand the appearance and contents of the visible universe.
Stars are born out of cold dense clouds of gas and dust. The process begins when a nearby disturbance compresses the gas, such as a supernova explosion or a shock wave from a black hole. The compression allows gravity to work, drawing more matter in to make a protostar and a disk of spinning matter. Planets are born from that disk, while the protostar gathers enough mass to begin nuclear fusion. Astronomers hunt for these infant stars and their protostellar disks, to identify the processes and particular types of atoms involved.
Oftentimes, many stars are born from the same nebula, and remain close together in a star cluster. For that reason, many of the stars in a cluster have similar chemistry and were born at roughly the same time. Some clusters contain very old stars, while others are far younger, providing astronomers with a laboratory for understanding stellar evolution.
Stars spend most of their lives on the main sequence, where they fuse hydrogen into helium in their cores. When the available fuel is used up, they swell into giants and go through another cycle of evolution. Astronomers track the way this works by studying the structure of the stars. The internal workings become visible through vibrations — starquakes — and magnetic cycles that produce fluctuations in the star’s light, as well as chemical changes on the surface. In addition, aging stars can pulsate, changing their brightness in various ways. Some of these stars, known as Cepheid variables, fluctuate predictably enough to be used for measuring distances to nearby galaxies.
Many stars have one or more companions, and those can affect a star’s life profoundly. Close binary stars can pull each other out of shape, strip matter from each other, or even merge into one star. If the companion is a white dwarf, neutron star, or black hole, interactions can be even more dire for the star. These can shorten the star’s life and sometimes produce small explosions on the its surface.
When a star in the same mass range as the Sun dies, it sheds its outer layers, forming a planetary nebula. The elements from those outer layers enrich the surrounding environment, while the shape of the planetary nebula itself provides clues to the final days of the original star. Meanwhile, the remnant of the star’s core becomes a white dwarf.
High-mass stars explode as supernovas, which are energetic enough to fuse more elements and spread them through space. The remnant of the star’s core shrinks into a neutron star for moderately high-mass stars, or a black hole for very high-mass stars. Astronomers study supernovas and their remnants to understand the way these stars die and spread materials through the galaxy. The neutron stars and black holes they leave behind also shape their environments in profound ways.
Artist's impressions of three types of supernovas from the explosion of extremely massive stars. Such models help us understand why some observed supernovas produce powerful jets, while others do not.