What we’ve learned after 32 years of NASA’s Hubble

On April 24, 1990, NASA launched the Hubble Space Telescope into Earth’s orbit.

This photo shows the Hubble Space telescope being deployed, on April 25, 1990, one day after its launch. It was taken by the IMAX Cargo Bay Camera (ICBC) mounted aboard the space shuttle Discovery. Originally launched to an altitude of ~620 km, Hubble is now about ~100 kilometers lower as of May 2024, and will continue its orbital decay due to atmospheric drag.

Originally, a flaw in the optics led to disappointingly blurry images.

The before-and-after difference between Hubble’s original view (left) with the mirror flaws, and the corrected images (right) after the proper optics were applied. The first servicing mission, in 1993, brought the true power of Hubble to the forefront of astronomy, where it’s remained ever since.

But subsequent servicing missions transformed Hubble into the epic observatory we all know.

Pluto, shown as imaged with Hubble in a composite mosaic, along with its five moons. Charon, its largest, must be imaged with Pluto in an entirely different filter due to their brightnesses. The four smaller moons orbit this binary system with a factor of 1,000 greater exposure time in order to bring them out. Nix and Hydra were discovered in 2005, with Kerberos discovered in 2011 and Styx in 2012. These five moons were likely formed via an early collision, rather than either in situ or as a result of gravitational capture.

As it has shown us the Universe, we’ve answered many of our deepest questions.

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

We didn’t know what was out there in the deepest depths of space.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint or too red or too obscured for Hubble to reveal, but when we extrapolate over the entire observable Universe, we expect to obtain a total of ~2 trillion galaxies.

We had never seen an infant galaxy before.

Only because the most distant galaxy spotted by Hubble, GN-z11, is located in a region where the intergalactic medium is mostly reionized, was Hubble able to reveal it to us at the present time, breaking the prior record held by EGSY8p7. Other galaxies that are at this same distance but aren’t along a serendipitously greater-than-average line of sight as far as reionization goes can only be revealed at longer wavelengths, and by observatories such as JWST. At present, GN-z11 has been relegated to the 9th most distant galaxy known as of 2024: in the JWST era.

We had no known instances of planets orbiting around stars other than the Sun.

The combination of Subaru data (red image) and Hubble data (blue image) reveals the presence of an exoplanet at a distance of 93 Astronomical Units (where 1 A.U. is the Earth-Sun distance) from its parent star. The luminosity of the massive object indicates reflected stellar emission rather than unimpeded direct emission, while the lack of a polarization signal is highly suggestive of a formation scenario other than core accretion. This is one of more than 5000 exoplanets presently known.

We didn’t know whether the Universe was 10 billion or 16 billion years old.

The light from any galaxy that was emitted after the start of the hot Big Bang, 13.8 billion years ago, would have reached us by today so long as it’s within about 46.1 billion light-years at present. But the light from the earliest, most distant galaxies will be blocked by intervening matter and redshifted by the expanding Universe. Both represent severe challenges to detection, which is why Hubble couldn’t see beyond about a redshift of 11, even under the most serendipitous circumstances. JWST has already broken that record.

We didn’t know whether space was expanding at 50 or 100 km/s/Mpc.

Although there are many aspects of our cosmos that all data sets agree on, the rate at which the Universe is expanding is not one of them. Based on supernovae data alone, we can infer an expansion rate of ~73 km/s/Mpc, but supernovae do not probe the first ~3 billion years of our cosmic history. If we include data from the cosmic microwave background, itself emitted very close to the Big Bang, there are irreconcilable differences at this moment in time, but only at the <10% level!

We didn’t know whether dark matter was hot, warm, or cold, or how much there was.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.

We didn’t know about the existence of dark energy or what the Universe’s fate would be.

The impressively huge galaxy cluster MACS J1149.5+223, whose light took over 5 billion years to reach us, is among the largest bound structures in all the Universe. On larger scales, nearby galaxies, groups, and clusters may appear to be associated with it, but are being driven apart from this cluster due to dark energy; superclusters are only apparent structures, but the largest galaxy clusters that are bound can still reach hundreds of millions, and perhaps even a billion, light-years in extent.

We didn’t even know whether or not black holes were real.

This tiny sliver of the GOODS-N deep field, imaged with many observatories including Hubble, Spitzer, Chandra, XMM-Newton, Herschel, the VLT, and more, contains a seemingly unremarkable red dot. That object, a quasar-galaxy hybrid from just 730 million years after the Big Bang, showcases how bright and powerful quasars can be. Many of the “little red dots” seen by JWST and other observatories are brightness-enhanced by the activity of the central black hole, with some jets pointing directly along our line-of-sight.

After 32 years of Hubble, these questions and more have all been definitively answered.

The visible/near-IR photos from Hubble show a massive star, about 25 times the mass of the Sun, that has winked out of existence, with no supernova or other explanation. Direct collapse is the only reasonable candidate explanation, and is one known way, in addition to supernovae or neutron star mergers, to form a black hole for the first time. Direct collapse, while still under investigation, may be triggered by a stellar companion.

The frontiers have been pushed back, and now we seek to answer the follow-up questions.

In this comparison view, the Hubble data is shown in violet, while ALMA data, revealing dust and cold gas (which themselves indicate star-formation potential), is overlaid in orange. With its views out beyond the limits of infrared astronomy but sensitive to spectroscopic features, ALMA can detect some of the most distant ionized/excited elements in cosmic history.

Thank you, Hubble, and may ALMA, the JWST, and more continuously advance our neverending quest for knowledge.

The very first finely-phased image ever released by NASA’s James Webb Space Telescope shows a single image of a star, complete with six prominent diffraction spikes (and two less-prominent ones), with background stars and galaxies revealed behind it. The background galaxies were a surprise to astronomers; JWST is imaging the Universe at roughly double the performance precision it was design-specified for. Even images such as this, not originally designed for scientific purposes, may prove useful to astronomers studying the Universe as a unique and unexpected source of data.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.