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 that throw out gas and dust into the area, such as a supernova—the death and explosion of a massive star—which leaves behind a supernova remnant (see Crab nebula). Others are form by the cooling and condensation of existing material in the interstellar medium.

Other 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 of hydrogen. New, massive stars ionizes the surrounding gas in the nebula, and make the nebula visible at optical wavelengths (see Orion Nebula).

Famous and interesting nebulae

Crab Nebula

The Crab Nebula is a supernova remnant. The supernova event itself is most likely observed by Chinese astronomers in 1054. A neutron star lies at the centre of the nebula today.

(Right: Mosaic from multiple images taken with the Hubble Space Telescope)

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 SUO.

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

Cat's Eye Nebula

The Cat's Eye Nebula is a planetary nebula that was formed from the death of a Sun-like star.

Ring Nebula

NASA's Hubble Space Telescope has captured the sharpest view yet of the most famous of all planetary nebulae: the Ring Nebula (M57). In this October 1998 image, the telescope has looked down a barrel of gas cast off by a dying star thousands of years ago. This photo reveals elongated dark clumps of material embedded in the gas at the edge of the nebula; the dying central star floating in a blue haze of hot gas. The nebula is about a light-year in diameter and is located some 2000 light-years from Earth in the direction of the constellation Lyra.

The colors are approximately true colors. The color image was assembled from three black-and-white photos taken through different color filters with the Hubble telescope's Wide Field Planetary Camera 2. Blue isolates emission from very hot helium, which is located primarily close to the hot central star. Green represents ionized oxygen, which is located farther from the star. Red shows ionized nitrogen, which is radiated from the coolest gas, located farthest from the star. The gradations of color illustrate how the gas glows because it is bathed in ultraviolet radiation from the remnant central star, whose surface temperature is a white-hot 120,000 degrees Celsius (216,000 degrees Fahrenheit).

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 these claims are currently in dispute.

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.

Beehive Cluster

The Beehive Cluster is an open cluster similar to Hyades in its composition of stars.
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 resolve 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, a whole different world on their own.

Each galaxy contains its own innumberable stars, its own countless nebulae, star clusters, and planets. It is believed that a supermassive black hole lies at the centre 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 we are in, 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 just one of the many worlds to exist in this vast cosmic 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 merely 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. The Andromeda Galaxy is slightly larger than the Milky Way, with a diameter of 220,000 light-years, and has roughly 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 centre of Centaurus A lies a black hole that is actively churning material around 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 centre 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 distribution of the stars go from high-to-low as we go out from the core to edges, the intermediate area between the core and its outer edges 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. This kind of planets—non-existent in the Solar System—are now called “Hot Jupiters” 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 field of study belonging to that of astrobiology.

There are various ways that we can detect these alien worlds. The first 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. Proxima Centauri b also lies within the habitable zone of its host star. Any desire to migrate to the planet, however, must be met with disappointment. Red dwarfs are incredibly erratic in that they produce powerful stellar winds 2000x more intense than what Earth receives from solar winds. Additionally, any planet close enough to be near a red dwarf's habitable zone must be tidally locked. Such tidally-locked planets most likely suffer from the same fate as our moon, having very hot daytime and extremely cold nights. It is not likely that life can exist on such planets.

Kepler-452b (or Earth 2.0)

Kepler-452b is a rocky planet (likely) orbiting a Sun-like star. It lies within the habitable zone of its host star. Because of its distance, at ~1400 light-years away, it is difficult to make better observations with our current tools. 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.

51 Pegasi b

51 Pegasi b is the first ever exoplanet discovered. It is 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.
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 stopped 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 centre.

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. It has a mass between 1.4 and 3 solar masses, but 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 will 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. What happens if you have an object that is so massive that light—the unbreakable champion of speed in the Universe that travels at 299,792,458 m/s—can not escape the object? These objects exist, and are known as black holes.

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 centre of galaxies accelerate matter near its event horizon and spew them out as relativistic jets rather 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 living things: they are born in nurseries (nebulae); attain adulthood (main sequence), metabolises food (stellar nucleosynthesis); reach a cranky old stage (giant phase); and eventually dies (planetary nebula or supernova). Like living things, when stars die, they too leave behind stellar corpses. Depending on the initial mass of the star, stars leave behind different kinds of corpses: white dwarfs, neutron stars, or black holes.

These objects are the most extreme objects in the Universe, pushing the boundaries of our understanding of physics the more we study them. White dwarfs and neutron stars, for example, are made of an 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 centre of a black hole is a knowledge hitherto forbbiden to the human intellect.

High Energy Phenomenon
Core-Collapse Supernovae

Core Collapse Supernova

A core collapse supernova is what happens when stars much more massive than the sun die. While massive stars can sustain fusion react 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. Unable to sustain in 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 (thermonuclear) supernovae are the explosions of white dwarfs. It is hypothesised that a Type Ia supernova begins with a binary system of a white dwarf and a companion star. Due to certain astrophysical mechanisms, the white dwarf starts accreting matter from the companion star. As white dwarfs have a mass limit of about 1.4 solar masses, white dwarfs that accrete matter from than their mass limit will lose its structural stability and go through a thermonuclear explosion. The explosion creates huge quantities of 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, though not as catastrophic as a type Ia supernova.
Kilonovae

Kilonovae

A kilonova occurs when two neutron stars merge. 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 as 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.

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 realised in the form of gravitational waves, a distruption 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.