In the vast universe, galaxies are huge collections of gas, dust, and stars, often containing hundreds of billions of stars. Our home galaxy, the Milky Way, is a beautiful spiral galaxy with long, winding arms. Galaxies come in different shapes, like elliptical, irregular, or peculiar. Besides their shapes, galaxies are also classified by their behavior, location, and mass.
In the 1960s, astronomers found something strange called 3C273. It looked like a regular blue star through optical telescopes, but radio telescopes showed it was an extremely bright galaxy. This galaxy, over 2 billion light-years away, emitted more energy than anything else known at the time—over 4 trillion times the energy output of the Sun. This led to the term “quasi-stellar radio source,” or “quasar,” describing these powerful cosmic objects.
The discovery of 3C273 led to finding more similar objects. With better X-ray observatories, astronomers found even more energetic galaxies, some emitting gamma rays. These galaxies are called “active galaxies,” classified by their light emission and spectral features.
The huge energy output of active galaxies comes from supermassive black holes at their centers. In the 1980s, astronomers suspected that large galaxies have these massive black holes, a theory later confirmed by observations. These black holes can be millions to billions of times the mass of the Sun.
As galaxies form, some material falls toward the center, feeding the black hole and helping it grow with its host galaxy. While black holes are known for consuming everything, including light, they can also power active galaxies. Matter falling into a black hole forms an accretion disk, heating up to millions of degrees due to friction and other forces. This intense heat causes the material to emit light across the electromagnetic spectrum, making active galaxies some of the brightest objects in the universe.
Active galaxies can look different depending on our viewing angle of their accretion disks. Depending on how we see them, we might observe different types of light, from infrared to X-rays. Some active galaxies also produce jets—powerful beams of matter and energy that can travel vast distances across the universe.
Our Milky Way galaxy has a supermassive black hole at its center, with a mass of over 4 million times that of the Sun. However, it is currently quiet, meaning it is not actively feeding and does not emit the intense energy typical of active galaxies. This might change in the distant future, especially due to potential galactic collisions.
The Milky Way is part of the Local Group, a collection of several dozen galaxies, including the Andromeda Galaxy. Andromeda is on a collision course with the Milky Way, moving at a speed of about 100 km/sec. While this collision is inevitable, it won’t happen for several billion years. When it does, the two galaxies will merge, likely forming a larger elliptical galaxy called “Milkomeda.”
Beyond the Local Group, galaxies are found in larger structures known as galaxy clusters. The nearest cluster, the Virgo Cluster, contains over a thousand galaxies and is about 50 million light-years away. Clusters can form superclusters, which are vast collections of multiple clusters. Our Local Group is part of the Virgo Supercluster, which might be a part of the even larger Laniakea Supercluster, containing around 100,000 galaxies.
The distribution of galaxies and clusters reveals a “cosmic web” structure, with galaxies forming along filaments and vast voids in between. This large-scale structure of the universe holds clues to fundamental questions about its composition, origin, and ultimate fate.
To estimate the total number of galaxies in the universe, astronomers used the Hubble Space Telescope to observe a seemingly empty patch of sky. The resulting Hubble Deep Field revealed thousands of galaxies in a tiny area, leading to the conclusion that there are approximately a hundred billion galaxies in the universe, each containing billions of stars.
The universe is a vast and complex place, filled with active galaxies powered by supermassive black holes. Our Milky Way is part of a larger cosmic structure, and while it may seem small in the grand scheme, our curiosity and quest for knowledge make us a significant part of this expansive cosmos. As we continue to explore and learn, we uncover the mysteries of the universe, stretching our understanding of existence itself.
Using online simulations or a drawing app, create representations of different galaxy shapes: spiral, elliptical, and irregular. Consider how these shapes might affect the behavior and evolution of galaxies. Share your creations with the class and discuss the potential reasons behind these diverse forms.
Calculate the energy output of a quasar compared to the Sun. Given that a quasar can emit over 4 trillion times the energy of the Sun, express this in terms of $E = mc^2$. Discuss how this immense energy is generated and what it tells us about the quasar’s environment.
Create a physical or digital model of a black hole and its accretion disk. Use materials like clay, paper, or 3D modeling software. Highlight how matter spirals into the black hole and emits light across the electromagnetic spectrum. Present your model to the class, explaining the processes involved.
Use a computer simulation to model the collision between the Milky Way and Andromeda galaxies. Observe how the galaxies interact over time and predict the outcome of their merger. Write a short report on your findings, including potential changes to the structure and star formation rates in the resulting galaxy.
Research the large-scale structure of the universe, focusing on the cosmic web. Create a visual representation of this structure, showing filaments and voids. Discuss how this arrangement influences galaxy formation and the distribution of matter in the universe.
Galaxies – Large systems of stars, interstellar gas, dust, and dark matter, bound together by gravity, often containing billions of stars. – The Milky Way and Andromeda are examples of spiral galaxies, each containing hundreds of billions of stars.
Quasars – Extremely luminous active galactic nuclei, powered by supermassive black holes at their centers, emitting vast amounts of energy. – Quasars are among the most distant and energetic objects in the universe, often outshining entire galaxies.
Black Holes – Regions of spacetime exhibiting gravitational acceleration so strong that nothing, not even light, can escape from them. – The event horizon of a black hole marks the boundary beyond which no information can return to the outside universe.
Energy – The capacity to do work or produce change, often measured in joules or electron volts in physics. – In astrophysics, the energy emitted by a star is a result of nuclear fusion occurring in its core.
Accretion – The process by which matter is accumulated onto a celestial body, such as a star or black hole, often forming an accretion disk. – The accretion of gas onto a black hole can result in the emission of X-rays and other high-energy radiation.
Light – Electromagnetic radiation that is visible to the human eye, and also refers to the broader spectrum of electromagnetic waves. – The speed of light in a vacuum is approximately $3 times 10^8$ meters per second, a fundamental constant in physics.
Jets – Narrow streams of particles and radiation ejected at high speeds from the poles of rotating astronomical objects, such as black holes or neutron stars. – The powerful jets emitted by quasars can extend for millions of light-years into intergalactic space.
Universe – The totality of all space, time, matter, and energy that exists, including galaxies, stars, and planets. – The observable universe is estimated to be about 93 billion light-years in diameter.
Clusters – Groups of galaxies bound together by gravity, often containing hundreds or thousands of galaxies. – The Virgo Cluster is one of the nearest galaxy clusters to the Milky Way, containing over a thousand galaxies.
Cosmic – Relating to the universe or cosmos, especially as distinct from the Earth. – Cosmic microwave background radiation provides evidence for the Big Bang theory and the early state of the universe.
