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What is a galaxy?

A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)

A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)

Galaxies are huge collections of stars, dust and gas. They usually contain several million to over a trillion stars and can range in size from a few thousand to several hundred thousand light-years across.

There are hundreds of billions of galaxies in the Universe.

Galaxies come in many different sizes, shapes and brightnesses and, like stars, are found alone, in pairs, or in larger groups called clusters.

Galaxies are divided into three basic types: spirals, ellipticals and irregulars.

– CoolCosmos, NASA,  IPAC (Infrared Processing and Analysis Center)

What types of galaxies are there?


Where is our solar system, within our galaxy?

When we look up at night, what part of our galaxy do we see?
Why does it appear that way?

Since our solar system lies in one of the spiral arms, we live in the flat plane of the Milky Way. We can actually see the dense plane of the Milky Way stretch across the sky in dark places that do not have a lot of surrounding light pollution. The diffuse light is the combined light from millions of stars. Some of the light from these stars are obscured by large clouds of dust, which is why there are dark patches. Dust and gas are necessary to form stars, and most stars are formed within the spiral arms. Note that we can’t really see the center of the galaxy with our eyes because there is dust in the way!

– http://www.astro.ucla.edu/~ghezgroup/gc/journey/journey_intro.html

Imaging the center of our galaxy

You’re looking at the center of our galactic home, the Milky Way, as imaged by 64 radio telescopes in the South African wilderness.

The black hole itself is invisible, but as material is pulled inside, some of emits light that escapes. Also, not all material near a black hole actually enters it. Some is pulled around it, compressed and heated, and then flung it, all the time radiating various forms of radiation. So what we see is this escaping radiation.

black hole center of the Milky Way By SARAO

The black hole at the center of the Milky Way and filaments. Image: (Credit: SARAO)

You’re looking at the center of our galactic home, the Milky Way, as imaged by 64 radio telescopes in the South African wilderness. [This is part of the] MeerKAT radio telescope. But these scopes form part of an even more ambitious project: the Square Kilometer Array, a joint effort to build the world’s largest telescope, spanning the continents of Africa and Australia.

This image shows filaments of particles, structures that seem to exist in alignment with the galaxy’s central black hole. It’s unclear what causes these filaments. Maybe they are particles ejected by the spinning black hole; maybe they are hypothesized “cosmic strings;” and maybe they’re not unique, and there are other, similar structures waiting to be found, according to a 2017 release from Harvard-Smithsonian Center for Astrophysics.

“This image from MeerKAT is awesome to me because the fine filaments seen in the radio image are excellent tracers of the galactic magnetic field, something we don’t get to see in most optical and infrared data,” Erin Ryan, research space scientist at the NASA Goddard Space Flight Center.

Interstellar travel

How long would it take a spaceship to get from one star to another? To get to the center of our galaxy? (to be added)

Galaxies moving through the universe

Gravitational repulsion and the Dipole Repeller

Learning Standards

Massachusetts Curriculum Frameworks Science and Technology/Engineering (2016) 

6.MS-ESS1-5(MA). Use graphical displays to illustrate that Earth and its solar system are one of many in the Milky Way galaxy, which is one of billions of galaxies in the universe.

A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012)

By the end of grade 8. Patterns of the apparent motion of the sun, the moon, and stars in the sky can be observed, described, predicted, and explained with models. The universe began with a period of extreme and rapid expansion known as the Big Bang. Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe.

By the end of grade 12. The star called the sun is changing and will burn out over a life span of approximately 10 billion years. The sun is just one of more than 200 billion stars in the Milky Way galaxy, and the Milky Way is just one of hundreds of billions of galaxies in the universe.

Benchmarks: American Association for the Advancement of Science

The sun is a medium-sized star located near the edge of a disc-shaped galaxy of stars, part of which can be seen as a glowing band of light that spans the sky on a very clear night. 4A/M1a

The universe contains many billions of galaxies, and each galaxy contains many billions of stars. To the naked eye, even the closest of these galaxies is no more than a dim, fuzzy spot. 4A/M1bc

Some distant galaxies are so far away that their light takes several billion years to reach the earth. People on earth, therefore, see them as they were that long ago in the past. 4A/M2de

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