The galaxy cluster Abell 2813 (also known as ACO 2813) has an almost fragile elegance in this picture from the NASA/ESA Hubble Space Telescope, which also highlights the impressive mechanics at work within it. The picture exemplifies gravitational lensing in a stunning way.

There are many distinct crescent shapes in the tiny dots, spirals, and ovals that make up the cluster's galaxies. There aren't curved galaxies but curved light arcs. They are excellent examples of gravitational lensing, a process that occurs as light bends through an object.

When the gravity of an object forces light to bend, this is known as gravitational lensing. Light from galaxies outside Abell 2813 is visible as curved crescents and "S" shapes. The galaxy cluster serves as a gravitational mirror because of its mass, bending light from more distant galaxies surrounding it. These distortions can take on a variety of forms, as long lines or arcs.

This visual proof of Einstein's theory of general relativity that mass allows light to bend is well-known.

The picture is a composite of observations made with the Advanced Camera for Surveys and Large Field Camera 3 on the Hubble Space Telescope.

Hubble Space Telescope

Infographic: Calculating the Hubble Constant (IMAGE)
This illustration shows the three basic steps astronomers use to calculate how fast the universe expands over time, a value called the Hubble constant. All the steps involve building a strong "cosmic distance ladder," by starting with measuring accurate distances to nearby galaxies and then moving to galaxies farther and farther away. This "ladder" is a series of measurements of different kinds of astronomical objects with an intrinsic brightness that researchers can use to calculate distances. Among the most reliable for shorter distances are Cepheid variables, stars that pulsate at predictable rates that indicate their intrinsic brightness. Astronomers recently used the Hubble Space Telescope to observe 70 Cepheid variables in the nearby Large Magellanic Cloud to make the most precise distance measurement to that galaxy. Astronomers compare the measurements of nearby Cepheids to those in galaxies farther away that also include another cosmic yardstick, exploding stars called Type Ia supernovas. These supernovas are much brighter than Cepheid variables. Astronomers use them as "milepost markers" to gauge the distance from Earth to far-flung galaxies. Each of these markers build upon the previous step in the "ladder." By extending the ladder using different kinds of reliable milepost markers, astronomers can reach very large distances in the universe. Astronomers compare these distance values to measurements of an entire galaxy's light, which increasingly reddens with distance, due to the uniform expansion of space. Astronomers can then calculate how fast the cosmos is expanding: the Hubble constant. NASA, ESA and A. Feild (STScI)

In 1990, the Hubble Space Telescope was deployed into space aboard the space shuttle Discovery's payload bay. The key goal was to determine the Universe's distance scale (how deep it is) and the origins of the elements found in space. That is what scientists are interested in. But the photos it produces! That is what fascinates the majority of mankind.

Abell 2813 (ACO 2813)

Hubble Watches Cosmic Light Bend
Hubble Watches Cosmic Light Bend ESA/Hubble & NASA, D. Coe

Thousands of galaxies of various ages, colors, and sizes make up galaxy clusters.

They have a mass that is about one million billion times that of the Sun.

Galaxy clusters were once thought to be the largest structures in the Universe before superclusters, which usually contain thousands of galaxy clusters and classes and range hundreds of millions of light-years, were discovered in the 1980s.

On the other hand, clusters have one advantage: superclusters are not tied together by gravity, but galaxy clusters continue to hold the status as the Universe's largest gravity-bound objects.

Deep Space Exploration

Spaceship
Stefan Kunze on Unsplash

Extreme space exploration (also known as deep-space exploration) is a field of astronomy, astronautics, and space science concerned with reaching the far reaches of space.

However, there is no agreement about what "distant" regions represent. It may also be used to refer to outer space in some cases. According to the International Telecommunication Union, deep space begins at a depth of 2 million kilometers (roughly 0.01 AU) from the Earth's atmosphere. NASA's Deep Space Network has used standards ranging from 16,000 to 32,000 kilometers from Earth. Human spaceflights (deep-space astronautics) and robotic spacecraft also perform physical space exploration.

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