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

Crab Nebula

The Crab Nebula is a supernova remnant formed extremely recently in cosmic terms. The supernova itself was likely to be observed by Chinese astronomers in 1054, who reported a "guest star" so bright that it is visible in daylight for the next 23 days since its appearance. At the center of the nebula today lies a pulsar, a neutron star forged by the intense pressure in the final moments of a star's death, spinning 30.2 times per second about its axis.

(Right: Mosaic from multiple images taken with the Hubble Space Telescope; colors resemble but are not true to the eye)

Orion Nebula

The Orion Nebula is one the brightest nebulae in the night sky, and one of the most photographed objects in the night sky. The intense star-formation in the region results in turbulent motions in the gas, and photo-ionising effects on the gas by massive stars can be observed in narrowband filters with the Nieves Observatory.

(Right: Image mosaic of Visible, H-alpha, and near-IR bands from the Hubble)

Cat's Eye Nebula

Unlike the Crab Nebula, which is a supernova remnant, the Cat's Eye Nebula is a planetary nebula. A planetary nebulae is a glowing shell of ionized gas that was ejected from a red giant star in the twilight years of a Sun-like star. While most planetary nebulae are morphologically simple, the Cat's Eye Nebula is structurally complex, with concentric rings surrounding an inner core. The mechanism that gives rise to its structure is not well-understood, and is an ongoing area of research.

(Right: Image by the Hubble)

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

Omega Centauri

Omega Centauri is the largest and most massive globular cluster in the Milky Way. It consists of over 10 million stars and has a total mass of ~4 million solar masses. It is some evidence that Omega Centauri houses an intermediate-mass black hole at its core, but the matter is still in dispute in the scientific community.

Pleiades Cluster

Pleiades is a stunning, bright open cluster of very hot and luminous blue stars. As one of the brightest and nearest clusters to Earth, Pleiades Cluster is visible to the naked eye in a sufficiently dark sky.

Hyades

Hyades is an open cluster, and is the closest star cluster to Earth. It is roughly spherical and contains about a hundred stars. Viewed from Earth with the naked eye, it looks like a V-shape in the sky.
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).

Milky Way

The galaxy that we call home, the Milky Way, is a barred spiral galaxy with a diameter of about 200,000 light-years. Home to more than 100 billion stars and more than 100 billion planets, Earth is merely one of the many worlds to exist in this galactic bungalow. The Milky Way gets its name from the hazy band of light that we can see in the very dark sky, which is the view of the disk-shape structure from our Earth-bound perspective. Since it is not possible to take a picture of the Milky Way as a whole, the image on the right is a simulation of what we think the Milky Way looks like.

Andromeda Galaxy

The Andromeda Galaxy is the sister galaxy of the Milky Way and is the closest galaxy to us. With a diameter of 220,000 light-years, the Andromeda Galaxy is slightly larger than the Milky Way, but has 2-3 times more stars than the Milky Way. The Andromeda Galaxy is expected to collide with the Milky Way in about 4.5 billion years. The two galaxies are expected to merge into a giant lenticular galaxy called Milkdromea.

Centaurus A

At the center of Centaurus A lies a supermassive black hole actively churning material around it and releasing relativistic jets. The image on the right is a composite of optical and x-ray photographs, exhibiting the structure of the relativistic jet ejected from the center of the galaxy.

Sombrero Galaxy

The Sombrero Galaxy is a lenticular galaxy about 30% the size of the Milky Way. Most prominent is the dust lane around the galaxy, which gives it an appearance of a sombrero. Personally, I think it looks more like a UFO.

Hoag's Object

A galaxy so weird in appearance that it doesn't even have the word "galaxy" in its name, the Hoag's Object is a non-typical galaxy called a ring galaxy. Unlike most other galaxies where the density of stars falls off gradually from the core to edges, the intermediate area between the core and outer edges of the Hoag's Object appears to be empty. Even weirder is how there is another ring galaxy within the rings of the Hoag's object (at about 1 o'clock in the image). The morphology of the galaxy is not quite in line with our theories of galaxy formation and remains an intriguing galaxy for study.
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

Proxima Centauri b

Proxima Centauri b is the closest known exoplanet to Earth orbiting the red dwarf star Proxima Centauri, a star that is part of a three-star system. More amazingly, Proxima Centauri b lies within the habitable zone of its host star. Any ambition to migrate to the planet, however, must be met with disappointment. Red dwarfs are known to be incredibly erratic, regularly producing stellar winds 2000x more intense than the solar winds Earth receives from the Sun. Additionally, any planet close enough to be near a red dwarf's habitable zone are tidally locked. Such tidally-locked planets likely suffer the same fate as our Moon, having very hot daytime and extremely cold nights. It is highly unlikely humans can call this place a second home.

Kepler-452b (or Earth 2.0)

Kepler-452b, also known as Earth 2.0, is a rocky planet orbiting a Sun-like star and lies within the habitable zone of its host star. Because of it is so far away, it is difficult to make better observations of the exoplanet with our current tools. In fact, we cannot even be sure that it exists! It is hoped that the James Webb Space Telescope, with its ability to observe objects in the deep infrared, can shed more light on this analog of Earth. For space-traveller hopefuls, it will take about 26 million years to reach this planet going at the speed that the spacecraft New Horizon does.

51 Pegasi b

51 Pegasi b is the first ever exoplanet discovered and also the first discovered class of planets we now call Hot Jupiters. Although it has a much lower mass than Jupiter, it is probably bigger in size, as its atmosphere is superheated (surface temperature of ~1000°C) by its host star. It is likely that the planet was formed much farther away from its host star than from where it is today, and migrated towards its host star through one of a few possible migration mechanisms.
Dead Stars

White Dwarf

When an average star like our Sun runs out of fuel, nuclear fusion halts and the stellar equilibrium is disrupted. The star begins to collapse unto itself under its own weight, before the whole process is halted once again by the formation of a white dwarf. The collapsing material bounces back out and is ejected into space, creating a planetary nebula and slowly exposes the white dwarf at its center.

A white dwarf is made of electron-generate matter and is prevented from further collapse due to the pressure from fast-moving electrons on its surface. These objects are roughly the size of Earth and have a mass limit of 1.4 solar masses—any more mass and the electrons will not be able to sustain the pressure, causing them to go supernova.

White Dwarf

Neutron Star

Not a star per se, but really another type of stellar corpse. Neutron stars are created following the death of stars that much larger than the sun. During the collapse, the electrons and protons in the soup of material are fused through inverse beta decay, forming neutrons in the end.

Neutron stars are about the densest objects there are in the Universe, almost as dense as an atomic nucleus. They have a mass between 1.4 and 3 solar masses, but are only about 20km in diameter. Neutrons stars also have powerful magnetic fields that induces powerful beams of radiation around its magnetic pole. When oriented in the direction towards Earth, we observe these regular pulses of radiation as the neutron star spins about its axis. These kind of neutron stars are also known as pulsars.

Neutron Star

Black Hole

All astronomical objects have an escape velocity, the speed required for an object to escape the gravitational influence from another. For Earth, the escape velocity is ~11.2 km/s, and that means you need a rocket that flies faster than 11.2km/s to leave the Earth. The higher the mass of an object, the greater its escape velocity. But what happens when you have an object that is so massive that light—which already travels at the cosmic speed limit—cannot escape the object? What you have is a black hole, an object with an escape velocity greater than the speed of light.

Despite its name, black holes are not perfectly black. They do emit radiation through a mechanism known as Hawking's radiation. We can also observe black holes, albeit indirectly, through from their interaction with their surroundings. Active black holes at the center of galaxies accelerate matter near its event horizon and spew them out as relativistic jets occasionally. Black holes also heat the material around them, providing themselves with a ring of luminous gas as we see in the picture of the black hole in M87 on the right, taken by the Event Horizon Telescope.

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

Core Collapse Supernova

A core collapse supernova is a violent explosion caused by the death of a massive star. While massive stars can sustain fusion reaction in its core beyond hydrogen fusion, stellar nucleosynthesis cannot proceed further than iron, as the energy required to fuse iron into a heavier element is greater than the energy that is produced from the fusion. As the star is thrown off its thermal equilibrium, the star goes through a titanic explosion, releasing all of its material in the vicinity. A neutron star or a black hole is usually left behind from a core collapse supernova.
Type Ia Supernovae

Type Ia Supernova

Type Ia supernovae are thermonuclear explosions caused by an ignition of white dwarfs that exceed a fixed critical mass. These white dwarfs are usually in a binary system, where they accrete matter from their companion up to the point of structural instability. The white dwarf then undergoes a runaway reaction, resulting in a supernova and releasing copious amounts of energy that obliterates the initial system.

Type Ia supernovae produces a consistent peak brightness because of this fixed critical mass at which a white dwarf will explode. This makes them excellent candidates as standard candles to measure distances in, and ultimately the age of, the Universe. The explosion also creates abundantly the isotope nickel-56 which decays to cobalt-56 and finally to the stable iron-56. Most of the iron in the Universe is created from Type Ia supernovae.

Classial Novae

Classical Novae

Nova—"new" in Latin—were once thought to be new stars in the night sky. It is now known to be white dwarfs that are experiencing a thermonuclear explosion its surface caused by mass accretion, though not as catastrophic as a Type Ia supernova.
Kilonovae

Kilonovae

A kilonova occurs when two neutron stars merge. Astronomical objects in a binary system are usually stable and do not fall into each other, but in the case of neutron star binaries, the intense gravitational effects of orbiting neutron stars radiate their gravitational bond off in the form of gravitational waves, in a mechanism known as gravitational decay. They eventually crash into each other and result in a rare event known as a kilonova. While they are more than a thousand times brighter than a classical nova, they are only about 1-10% as bright as a supernova and are not as easy to find.

Kilonova produces almost all of the extra-heavy and extra-rare elements in the Universe, such as gold, silver, and platinum. A merger between a neutron star and a black hole is also called a kilonova.

Black Hole Mergers

Black Hole merger

The mergers of black holes are the most energetic events observable in the Universe, but they are also invisible. Black hole mergers can only be detected by gravitational waves; the first of such merger was detected in 2015. The event in 2015, formally known as GW150914, produced three solar masses of energy in the form of gravitational waves in its final 20ms of merger, more energy than the combined electromagnetic energy of the observable universe put together.

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.

© 2020 Nieves Observatory at Soka University of America.
With media resources from NASA & ESO/Hubble.