“I don’t think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I’m an optimist. We will reach out to the stars.” — Stephen Hawking, theoretical physicist and cosmologist.

Our Milky Way Galaxy: How Big is Space?

When we talk about the enormity of the cosmos, it’s easy to toss out big numbers – but far more difficult to wrap our minds around just how large, how far, and how numerous celestial bodies really are.
To get a better sense, for instance, of the true distances to exoplanets – planets around other stars – we might start with the theater in which we find them, the Milky Way galaxy.

“I don’t think the human race will survive the next thousand years, unless we spread into space. There are too many accidents that can befall life on a single planet. But I’m an optimist. We will reach out to the stars.” — Stephen Hawking, theoretical physicist and cosmologist.

Galaxies

Our galaxy, the Milky Way, is typical: it has hundreds of billions of stars, enough gas and dust to make billions more stars, and at least ten times as much dark matter as all the stars and gas put together. And it’s all held together by gravity.
Like more than two-thirds of the known galaxies, the Milky Way has a spiral shape. At the center of the spiral, a lot of energy and, occasionally, vivid flares are being generated. Based on the immense gravity that would be required to explain the movement of stars and the energy expelled, the astronomers conclude that the center of the Milky Way is a supermassive black hole.
Other galaxies have elliptical shapes, and a few have unusual shapes like toothpicks or rings. The Hubble Ultra Deep Field (HUDF) shows this diversity. Hubble observed a tiny patch of sky (one-tenth the diameter of the moon) for one million seconds (11.6 days) and found approximately 10,000 galaxies, of all sizes, shapes, and colors. From the ground, we see very little in this spot, which is in the constellation Fornax.

Hubble Ultra Deep Field galaxies:
Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team

 

Formation

After the Big Bang, the Universe was composed of radiation and subatomic particles. What happened next is up for debate - did small particles slowly team up and gradually form stars, star clusters, and eventually galaxies? Or did the Universe first organize as immense clumps of matter that later subdivided into galaxies?

Collisions

The shapes of galaxies are influenced by their neighbors, and, often, galaxies collide. The Milky Way is itself on a collision course with our nearest neighbor, the Andromeda galaxy. Even though it is the same age as the Milky Way, Hubble observations reveal that the stars in Andromeda's halo are much younger than those in the Milky Way. From this and other evidence, astronomers infer that Andromeda has already smashed into at least one and maybe several other galaxies.

Recent Discoveries

Date	Discovery
February 20, 2020	Beyond the Brim, Sombrero Galaxy's Halo Suggests a Turbulent Past
February 3, 2020	Nature's Grand Design (NGC 5364)
January 28, 2020	Chandra Spots a Mega-Cluster of Galaxies in the Making
January 27, 2020	Bars and Baby Stars (NGC 7541)
January 16, 2020	XMM-Newton Discovers Scorching Gas in Milky Way's Halo
January 6, 2020	Hubble Surveys Gigantic Galaxy (UGC 2885)
January 5, 2020	New View of Milky Way’s Center
January 5, 2020	Distant Galaxy Group Driving Ancient Cosmic Makeover

What is a galaxy, anyway?

Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe. Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure. Our galaxy probably contains 100 to 400 billion stars, and is about 100,000 light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. Another galaxy, IC 1101, spans as much as 4 million light-years.

Observation data
Type	Sb, Sbc, or SB(rs)bc  (barred spiral galaxy)
Diameter	Stellar disk: ≈120 kly (37 kpc) Dark matter halo: ≈1.9 ± 0.4 Mly (580 ± 120 kpc)
Thickness of thin stellar disk	≈2 kly (0.6 kpc)
Number of stars	100–400 billion [(1–4)×1011]
Mass	(0.8–1.5)×1012 M☉
Angular momentum	≈1×1067 J s
Sun's distance to Galactic Center	25.6–27.1 kly (7.86–8.32 kpc)
Sun's Galactic rotation period	240 Myr
Spiral pattern rotation period	220–360 Myr
Bar pattern rotation period	100–120 Myr
Speed relative to CMB rest frame	552.2±5.5 km/s
Escape velocity at Sun's position	550 km/s[10]
Dark matter density at Sun's position	0.0088+0.0024 −0.0018 M☉pc−3 or 0.35+0.08 −0.07 GeV cm-3

Ok, fine, but what the heck is a light-year?

Glad you asked. It’s one of the most commonly used celestial yardsticks, the distance light travels in one year. Light zips along through interstellar space at 186,000 miles (300,000 kilometers) per second (more than 66 trips across the entire United States, in one second). Multiply that by all the seconds in one year, and you get 5.8 trillion miles (9.5 trillion kilometers). Just for reference, Earth is about eight light minutes from the Sun. A trip at light speed to the very edge of our solar system – the farthest reaches of the Oort Cloud, a collection of dormant comets way, way out there – would take about 1.87 years. Keep going to Proxima Centauri, our nearest neighboring star, and plan on arriving in 4.25 years at light speed.
If you could travel at light speed. Which, unless you’re a photon (a particle of light), you can’t, and, by current physics, might never be possible. But I digress.

Unit of                                        : length

1 ly in...                                      : is equal to

Metric (SI) units                         : 9.4607×10¹⁵ m; 9.4607 Pm

Imperial & US units                   : 5.8786×10¹² mi

Astronomical units                      : 63241 au   ; 0.3066 pc

Unit system                                 :  astronomy units

Symbol                                        : ly

Major components of the Galaxy

 

Star clusters and stellar associations

 Although most stars in the Galaxy exist either as single stars like the Sun or as double stars, there are many conspicuous groups and clusters of stars that contain tens to thousands of members. These objects can be subdivided into three types: globular clusters, open clusters, and stellar associations. They differ primarily in age and in the number of member stars.

our nearest galaxy andromeda?

 The Andromeda Galaxy (M31) is the closest spiral galaxy to us, and though it's gravitationally bound to the Milky Way, it's not the closest galaxy by far – being 2 million light years away. Andromeda is currently approaching our galaxy at a speed of about 110 kilometers per second.

 

Early observational history

In 964, the Persian astronomer Abd al-Rahman al-Sufi described the galaxy as a "small cloud" in his "Book of Fixed Stars," the first known report of our nearest neighbor. When Charles Messier labeled it M31 in 1764, he incorrectly credited the discovery of what was then called a nebula to the German astronomer, Simon Marius, who provided the first telescopic observation of the object. The first photographs of Andromeda were taken in 1887, by Isaac Roberts.

This image of the Andromeda Galaxy is a composite of an infrared photo from ESA's Herschel space telescope and the XMM-Newton’s X-ray telescope. The infrared frame shows rings of dust that trace gaseous reservoirs where new stars are forming and the X-ray image shows stars approaching the ends of their lives. (Image credit: ESA/Herschel/PACS/SPIRE/J.Fritz, U.Gent/XMM-Newton/EPIC/W. Pietsch, MPE)

In the 1920s, the distant galaxy became part of the Great Debate between American astronomers Harlow Shapley and Heber Curtis. At the time, astronomers thought the Milky Way composed the whole universe, and the strange patches known as nebulae lay inside of them. Curtis had spotted various novae in Andromeda, and argued instead that it was a separate galaxy.
The discussion wasn't concluded until 1925, when Edwin Hubble identified a special kind of star known as a Cepheid variable — a star whose characteristics allow for precise measurements of distance — within Andromeda. Because Shapley had previously determined that the Milky Way was only 100,000 light-years across, Hubble's calculations revealed that the fuzzy patch was too far away to lie within the Milky Way.
Hubble went on to use his measurements of the Doppler shifts of the galaxies to determine that the universe was expanding. The calculated distance to Andromeda doubled in the 1940s when Walter Baade was the first to observe individual stars in the central region of the galaxy, and found two different types of Cepheid variables. Radio maps of Andromeda followed in the 1950s, after radio emissions were detected by Hanbury Brown and Cyril Hazard at Jodrell Bank Observatory.