The Nieves Observatory presents

A Brief Guide to Astronomical Objects

BY JERRICK WEE

THE SOLAR SYSTEM

The Solar System was created 4.5 billion years ago, when the a cloud of molecular gas collapsed to form a proto-star—the star that which we today call the Sun. Along with the formation of the proto-star is the creation of an accretion disk around the star, which eventually coalesced to form the planets of the solar system.

The celestial bodies are among the brightest objects in the night sky, and many can be seen with the naked eye. The moon tops the list, and is followed by the five brightest planets in the solar system—in order of brightness: Venus (-4.3 mag), Jupiter (-2.2 mag), Mercury (0.08 mag) Mars (0.83 mag), and Saturn (1.43 mag).

These five planets are known since ancient times for their peculiar movements through the night sky. Because they are, unlike stars, not fixed in position in the night sky, the ancient Greeks called them planētai, or wanderers, from which the term “planet” is derived.

NEBULAE

A nebula (Latin for “cloud”) is an interstellar cloud of dust and gas. Some nebulae are created by cataclysmic events, such as the death and explosion of a massive star, that throw out gas and dust into the area. These powerful, destructive events leave behind a remnant that continues to glow from the residual heat of the explosion (see Crab nebula). Other nebulae are formed by the cooling and condensation of existing material in the interstellar medium.

Many nebulae are star-forming regions, and are also known as stellar nurseries. Stars are born when the giant clouds of gas in the nebula clump up and accrete matter till they collapses under their own weight, igniting a nuclear fusion from the accreted hydrogen. New, massive stars ionize the surrounding gas in the nebula, and render the nebula visible at optical wavelengths.

FAMOUS AND INTERESTING NEBULAE

No stars are born alone. They are usually formed together in a star cluster—a group of stars that are bounded gravitationally. There are two kinds of star clusters: globular clusters and open clusters. Globular clusters are dense groupings of numerous stars that are spherically distributed (see Omega Centauri). Open clusters, on the other hand, are loosely bound together and tend to be young, bright stars. Over time, open clusters disperse to become individual stars.

Star clusters are important for distance calibration in astronomy, and are especially useful when stars in a cluster are plotted on a Hertzprung-Russell (HR) diagram. As we can compare the position of the main-sequence for different clusters, this main-sequence fitting allows us to determine the distance of different kinds of clusters through the distance modulus.

FAMOUS AND INTERESTING STAR CLUSTERS

STAR CLUSTERS

GALAXIES

The Milky Way—that is, our Galaxy—was once considered to be all there is to the Universe. Everything we can see in the sky was considered part of the Milky Way; there is no “outside” of our Galaxy. We could see other celestial objects that we know today are other galaxies, such as the Andromeda Galaxy, but because they look like bodies of dust and clouds, they were thought be one of the many nebulae in our Galaxy.

Better telescopes eventually allow astronomers to see that galaxies are made of huge agglomerations of stars and are categorically distinct from nebulae. In 20th century, with even better telescopic evidence and distance calibration techniques, we learn that these “nebulae” are in fact distant galaxies themselves, whole different worlds on their own.

Each galaxy contains its own innumerable stars, its own countless nebulae, star clusters, and planets. It is believed that a supermassive black hole lies at the center of every galaxy. Morphologically, galaxies are thought to start out as elliptical galaxies and over time become spiral galaxies. However, there are some galaxies that do not conform to this galactic evolution, such as irregular galaxies and ring galaxies (see Hoag’s Object).

EXOPLANETS

Once a topic of intense speculation, exoplanets—planets that orbit other stars—are now a scientific certainty. The first exoplanet, 51 Pegasi b, is a Jupiter-like exoplanet so close to its host star that it orbits the star in just 4 days. These “Hot Jupiters” are non-existent in the Solar System, and are now known to be relatively common in the Galaxy. More interestingly, we have also found Earth-like exoplanets in the habitable zone of their host stars with water in their atmosphere. This opens up the possibly for alien life similar to that of Earth, a domain of study belonging to astrobiology.

There are various ways that we can detect these alien worlds. The first method used is the radial-velocity method, where the gravitational influence of an exoplanet is strong enough to induce a detectable motion in its host star. Another more widely-used method today is transit photometry, where an exoplanet partially obscures the brightness of its host star whenever it moves in front of the star in our line of sight from Earth. The transit method can be performed at our very own Nieves Observatory (see photometry tutorial).

NOTABLE EXOPLANETS

DEAD STARS

WHITE DWARF

NEUTRON STAR

BLACK HOLE

Stars are often described by astrophysicists as if they are living things: they are born in nurseries (nebulae); metabolises food (stellar nucleosynthesis); attain adulthood (main sequence); become elderly and cranky (giant phase); and eventually die (planetary nebula or supernova). Like living things, when stars die, they too leave behind corpses. Depending on the initial mass of the star, stars leave behind different kinds of stellar corpses: white dwarfs, neutron stars, or black holes.

Stellar corpses are the most extreme objects in the Universe, pushing the boundaries of our understanding of physics the more we study them. For example, white dwarfs and neutron stars are made of a little-studied, exotic substance called degenerate matter. And because of our conflicting understanding of what happens when something extremely massive occupies an infinitely small space, the mechanics at the center of a black hole is a piece of knowledge hitherto forbidden to the human intellect.

HIGH ENERGY PHENOMENON

CORE-COLLAPSE SUPERNOVAE

TYPE IA SUPERNOVAE

CLASSIAL NOVAE

KILONOVAE

BLACK HOLE MERGERS

When we say high energy in space, we really do mean it. An explosion of a white dwarf, called a type Ia supernova, shines with the intensity of 10 billion suns and produces an amount of energy equivalent to the amount that the Sun produces in its lifetime.

A black hole merger generates no light even though the energy output is much greater than that of a supernova. Most of the energy is released in the form of gravitational waves, a disruption of the fabric of spacetime propagating through the cosmos.

Most interesting are the phenomena that release energy in various forms. A kilonova—the merger of two neutron stars—produces both detectable gravitational waves and electromagnetic radiation, which allows astronomers to study such a phenomenon with both optical telescopes and gravitational wave observatories.

Multi-messenger astronomy is currently one of the newest fields in astronomy, and telescopes that can observe these transients (such as the Nieves Observatory) are well-positioned to advance the frontiers of astronomy and human knowledge.

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