This long-exposure image shows a number of bright stars, star-forming regions, and the plane of the Milky Way above the ALMA observatory in the Southern Hemisphere. The closest stars are only a few light-years away: less than a factor of 10 from the edge of the Oort Cloud. However, more distant stars and objects still visible to the naked eye can be tens of thousands of light-years away.

Behind the dome of a series of telescopes at the European Southern Observatory, the Milky Way rises in the southern sky, flanked by the Large and Small Magellanic Clouds (right). Although several thousand stars and the plane of the Milky Way are visible to the human eye, there are only four galaxies other than our own that the normal human eye can see without any aids.

This comparison image, showing the same region as captured by Hubble’s eXtreme Deep Field (top) and JWST’s JADES survey (bottom), shows a selection of many ultra-distant galaxies found in the early Universe. When we observe the Universe from a great distance, we see it as it was in the distant past: smaller, denser, hotter, and less evolved. Back at the limits of JWST’s capabilities, we see evidence of stars and galaxies everywhere.

The European Space Agency’s space-based Gaia mission has mapped the three-dimensional positions and locations of more than a billion stars in our Milky Way: more than ever before. The ability to measure stellar parallax, how a star’s real (rather than apparent) position changes over the course of a calendar year, is made much easier by superior instruments, large apertures, Gaia’s position in space, and the development of photography and computer-assisted identification of the relative displacement of stars. Over several years, Gaia can also provide information about the velocities of these stars.

This color-coded map shows the heavy element abundances of more than 6 million stars in the Milky Way. Stars in red, orange, and yellow are all so rich in heavy elements that they should have planets; green and cyan-coded stars should rarely have planets, and blue or violet-coded stars should have absolutely no planets around them. Note that the central plane of the galactic disk, which extends into the galactic core, has the potential for habitable, rocky planets. This map shows less than 0.01% of the stars in our galaxy.

There are numerous galaxies comparable to the Milky Way today, but younger galaxies similar to the Milky Way are inherently smaller, bluer, and generally more gaseous than the galaxies we see today. Over time, many smaller galaxies become gravitationally bound together, resulting in mergers, but also groups and clusters with a large number of galaxies overall.

Zw II 96 in the constellation Delphinus (Dolphin) is an example of a galaxy merger occurring about 500 million light-years away. These types of events trigger star formation and can consume large amounts of gas in each of the progenitor galaxies, as opposed to a steady stream of low-level star formation as seen in isolated galaxies. Note the streams of stars between the interacting galaxies, which may either become part of a stellar population in the post-merger galaxy’s stellar halo or be ejected entirely from the post-merger galaxy and migrate through the intergalactic medium. The end result will be a larger number of stars bound together in a smaller number of galaxies.

This section of one of our deepest views of the Universe, taken with JWST, overlaps with data from the Hubble eXtreme Deep Field. Compared to Hubble, JWST reveals a tremendous number of objects previously invisible to Hubble, even at only about 4% of observing time. Most of these galaxies are small and low-mass, but are currently forming stars rapidly, allowing JWST to detect their presence.

When large mergers of similar-sized galaxies occur in the Universe, they form new stars from the hydrogen and helium gas present within them. This can lead to greatly increased rates of star formation, similar to what we observe in the nearby galaxy Henize 2-10, located 30 million light-years away. This galaxy will likely evolve into another disk galaxy after the merger if large amounts of gas remain within it, or into an elliptical galaxy if all or almost all of the gas is ejected by the current starburst. Starburst events like this were much more common earlier in cosmic history than they are today.

The star formation history of the Universe reconstructed by the Fermi-LAT collaboration was compared with other data points from alternative methods found in the literature. We arrive at a consistent set of results across many different measurement methods, with the largest uncertainties at the highest redshifts and earliest times. These uncertainties represent less than 1% uncertainty in the total number of stars formed over the course of cosmic history.

This view of the Perseus galaxy cluster shows over 1,000 galaxies all clustered together at a distance of about 240 million light-years. Many tens of thousands more are visible in the background of the image. Although the image is visually dominated by the most massive galaxies with the most stars, these are much smaller, fainter, and less massive, making them extremely difficult to see even up close.

This deep region of the GOODS South field contains 18 galaxies where stars are forming so rapidly that the number of stars will double in just 10 million years: just 0.1% of the lifetime of the Universe. The deepest views of the Universe, as provided by space telescopes, take us back to the early history of the Universe, when star formation rates were much higher than today, but less than 1% of the Universe’s total stars had already formed. Many of the most distant galaxies are in close proximity to other foreground galaxies, whose mass distorts and amplifies the light from background objects.

JADES-GS-z14-0, in the upper inset, is behind (and just to the right of) a closer, brighter, bluer galaxy. Only the power of incredible resolution spectroscopy, able to separate the two sources, has enabled the nature of this record-breakingly distant object to be determined. Its light dates from when the Universe was only 290 million years old: just 2.1% of its current age. JADES-GS-z14-1, just below, dates from when the Universe was about 300 million years old. Compared to large, modern galaxies, all early galaxies contain few stars.