ASTROPHYSICS AND ASTRONOMY

The Physics of the Universe

Astronomy is, at its heart, the scientific study of objects and events in the universe. It’s a rich science, divided into two major branches:

1. Observational astronomy, which is concerned with gathering as much information about objects in the universe as possible. Think of it as the data-gathering part of astronomy.
2. Astrophysics, which applies physics to explain the properties, interactions, and evolution of planets, stars, the interstellar medium, nebulae, galaxies, and other objects in the distant reaches of the cosmos. Astrophysicists also apply aspects of chemistry, electromagnetism, particle physics, and other disciplines to explore and explain objects and processes in the universe.

Astronomers (who are also usually astrophysicists) make their observations using observatories outfitted with instruments sensitive to light from different parts of the electromagnetic spectrum. Using the data from their observations, these scientists come up with explanations for what’s happening in the universe. Theoretical astrophysics uses models, statistics, and simulations to explain objects in the universe and predict what they might do in the future.

Seeing the Light

Light is one of the most fundamental parts of the universe and is the standard “currency” in astrophysical research. Astronomers study light emitted and reflected from objects in order to understand more about them and their environments. Light can act as a particle, called a photon, or it can travel through space as a wave. This dual nature of light is central to how we detect objects in the universe. We can collect photons using cameras, but we can also measure wavelengths of light.

The word light is usually used to describe light our eyes can see. We evolved to be most sensitive to visible emissions from the Sun. But those are only a small part of the electromagnetic spectrum—the range of all possible light given off, absorbed, and reflected by objects in the universe. Most of the rest of the spectrum is invisible to us because it’s in the form of x-ray, ultraviolet, radio, infrared, and microwave emissions. Astronomers use specially sensitive instruments to detect them.

Infrared Astronomy

For centuries, astronomy was a visible-light science. In the 1800s, scientists began measuring and analyzing other wavelengths of light, starting with infrared, also known as thermal (heat) radiation. Anything that is even slightly heated gives off infrared radiation (often referred to as IR). Unaided, we may not be able to see what is doing the heating, but IR detectors can “lift the veil” for us.

A good example is a cloud of gas and dust surrounding a newborn star. IR-sensitive detectors zero in and show us the star. They let us see the region around a black hole or peer into the depths of a cloud hiding a star that’s about to die. Much infrared astronomy is best done from space, since Earth’s atmosphere absorbs a lot of incoming thermal radiation.

Ultraviolet Astronomy

Ultraviolet astronomy focuses on light that is more energetic than infrared or visible light. Ultraviolet (UV) is also absorbed by Earth’s atmosphere, so the best observations are done from space. What gives off UV in space? Hot and energetic objects do. This includes young stars and superheated interstellar gases. The Sun gives off UV, which is what burns your skin if you stay outdoors without good sunblock.

Radio and Microwave Astronomy

Early in the twentieth century, an engineer at Bell Labs named Karl Jansky (1905–1950) pointed a radio receiver at the sky and inadvertently became the first person to discover naturally occurring radio signals from an object in space. The emissions he found came from the center of the Milky Way Galaxy. Today, radio astronomers use vast arrays of radio dishes and antennas to detect signals from a wide variety of objects. These include superheated shells of plasma (energized gases) emanating from the cores of galaxies, shells of material from supernova explosions, and the microwave emissions from the vibration of interstellar molecules in clouds of gas and dust in interstellar space or in planetary atmospheres. In addition, atmospheric researchers use radars and radio dishes to study interactions of Earth’s upper ionosphere with the solar wind and radars to map such places as the cloud-covered surface of Venus and Saturn’s moon Titan.

X-Ray and Gamma-Ray Astronomy

The most energetic objects, events, and processes in the universe give off x-rays and gamma rays. These include supernova explosions such as the one that created Cygnus X-1, the first x-ray source to be discovered; high-speed jets of matter streaming from the cores of active galaxies; and distant powerful explosions that send both x-rays and gamma-ray pulses across space. To detect and study these powerful emitters, astronomers have used space-based observatories such as the Chandra X-Ray Observatory, the Roentgen Satellite (ROSAT), and XMM-Newton for x-rays, and the Compton Gamma-Ray Observatory (CGRO), the current Fermi mission, and the Swift satellite for detecting gamma-ray emissions in the universe.

One of the most startling discoveries in microwave astronomy came in 1964 when scientists Arno Penzias (1933–) and Robert W. Wilson (1936–) detected a glow of background radiation at microwave frequencies. It seemed to come from everything in the universe. This so-called “cosmic background radiation,” or CMBR, is the leftover glow from one of the earliest epochs of cosmic history shortly after the Big Bang occurred.

Spectroscopy in Astronomy

Astronomers pass light from an object through a specialized instrument called a spectroscope. Think of it as a very special type of prism, but instead of creating just the array of colors we see, it separates light into very fine divisions called a spectrum that extends far beyond what our eyes can detect. The chemical elements in an object emit or absorb wavelengths of light, which shows up in a spectrum as a glowing bar of light or a dark “dropout” line. Therefore, you can imagine spectra as cosmic bar codes; encoded in those bars is information about the chemical composition, density, mass, temperature, velocity, and other characteristics of planets, stars, nebulae, and galaxies.