The James Webb Space Telescope was launched into orbit on December 25th, 2021, after years of preparation and anticipation (what a Christmas present, huh?). The magnificent photographs and data it has provided since then have confirmed beyond a shadow of a doubt that it was the best Christmas present ever! After a year of operation, the JWST has met one of its key goals: to view the first stars and galaxies that formed in the Universe. The next-generation observatory has done so by breaking distance records and showing galaxies that lived less than 1 billion years after the Big Bang!
These research are critical for understanding the evolution of the universe and resolving problems with our cosmological models, such as the Hubble Tension and the mysteries of Dark Matter and Dark Energy. Hold on to your hats, for things have reached a whole new level of amazing! An international team of scientists has found a well-magnified star candidate in a galaxy that seems to be nearly 12.5 billion years old. The discovery of a star that existed when the Universe was only 1.2 billion years old demonstrates the JWST’s capabilities and provides a peek of what’s to come!
Lukas J. Furtak, a postdoctoral student in experimental astrophysics at Ben-Gurion University of the Negev, conducted the study. He led an international team of astronomers and astrophysicists from the Cosmic Dawn Center (DAWN), the Space Telescope Science Institute (STScI), the Association of Universities for Research in Astronomy (AURA), the Spanish National Research Council (CSIS), the Center for Extragalactic Astronomy, the Racah Institute of Physics, the Harvard-Smithsonian Center for Astrophysics (CfA), NASA’s Goddard Space Flight Center, and more.
Hubble and JWST observations of some of the Universe’s earliest galaxies have offered a wealth of information that has both questioned and supported current notions of cosmic evolution. Unfortunately, as the authors of the study pointed out, direct observation of individual stars at these distances is impossible because they are too weak in comparison to their surrounding galaxies. Scientists have shown that stars can be viewed through gravitational lensing, a process in which a big object in the foreground amplifies light from a distant source.
This phenomenon happens when the gravitational force of huge objects modifies the curvature of spacetime around them, as predicted by Einstein’s Theory of General Relativity. This approach has recently enabled astronomers to identify several dozen stars in powerful lensing star cluster environments, and the JWST has already discovered many. The scientists used photos from Webb’s Near-Infrared Camera (NIRCam), which captured the galaxy cluster MACS0647 during its first year of operation as part of the Cycle 1 General Observers (GO) program 1433, for their research.
Furtak told Universe Today via email that this was a significant achievement because lensed research have generally focused on high-redshift galaxies:
The study of individual lensed stars at cosmological distances is a relatively new field that has gained interest in recent years thanks to the phenomenal capacities of the Hubble and James Webb Space Telescopes. Individual stars can normally only be observed in our Galaxy and its immediate neighbours while at larger cosmological distances we only see whole galaxies.
“However, the gravitational lensing effect, where massive objects such as galaxy clusters deflect the light from background sources and magnify it, can change this if a single star in a lensed background galaxy happens to cross the so-called critical line which is a region where the gravitational magnification reaches extreme values. If the alignment is right, this then enables us to observe single stars in distant galaxies.Lukas J. Furtak
This large cluster’s gravity creates a powerful lens, which has already been utilized to discover the triple-lens JD galaxy, which has a redshift of z=11. This corresponds to an apparent distance of 13.4 billion light-years ago, implying that it looks today as it appeared fewer than 500 million years ago. Using the same lensing galaxy, the researchers acquired and studied spectra from an individual star at z=4.76 (MACS0647-star-1) at an apparent distance of roughly 12.35 billion years ago.
The star was discovered in 2022 using data from Webb’s NIRCam, as described in a publication by Dr. Ashish Meena of Ben-Gurion University (a colleague and co-author on this most recent paper). Furtak stated:
“[MACS0647-star-1] was identified as such due to its location in a strongly lensed and distorted background galaxy near or even on top of the critical line, i.e. in a region where the gravitational lensing magnification reaches extreme values.” MACS0647-star-2, a fainter second star, was also discovered in the same investigation. MACS0647-star-1 was discovered as a potential B-type supergiant star with a surface temperature of 100,000 K based on photometry in numerous broad-band filters.”
Furtak and his colleagues collected the MACS0647-star-1 spectra a few months later using Webb’s Near-Infrared Spectrometer (NIRSpec) as part of a bigger mission aimed at the entire lensing cluster. The spectra enabled scientists to precisely quantify the redshift to MACSO647-star-1, from which they calculated distance estimations indicating that the star existed when the Universe was only 1.2 billion years old. They also discovered that the spectrum presented a more complicated picture than the earlier photometric data, according to Furtak:
“While the imaging photometric measurement was consistent with a single B-type supergiant star, with the spectrum we now see, we must be looking at two stars – one B-type and one colder F-type – or at a hot B-type star whose light is reddened by dust somewhere along the line of sight.” However, the latter theory is more likely. That said, with the current spectrum – 1.8h integration time and NIRSpec-prism mode, which has a relatively low resolution – we can’t rule out the possibility that this isn’t a whole star-cluster instead of a single star (i.e., a globular-cluster type object, very dense old stellar population).”
Follow-up observations of the MACS0647 lensing galaxy are required in order to understand what Webb revealed in greater detail. Furtak specifically said that in order to detect absorption lines more precisely, much deeper spectra and far greater spectral resolution are required. Regardless, as Webb continues to investigate the stars and galaxies that were present in the early Universe, similar discoveries are expected to become widespread shortly. Hubble has so far found four lensed stars at cosmic distances; the first was Icarus, which it discovered in 2018, and the most recent was Earendel, which it discovered in 2022.
Furtak predicts that the JWST will locate lensed stars at a rate of one per observed galaxy cluster, based on what Webb has discovered in just its first year of observations. Numerous lensed stars, notably MACS0647-star-1, the second-farthest star recorded to date, have already been found. This, according to Furtak, provides a tantalizing sneak peek at what’s in store:
This study definitely shows that JWST has the instrumental capacity to not only detect lensed stars in imaging campaigns but to also obtain their spectra with NIRSpec. This is the second spectrum of a lensed star ever obtained and the first space-based one with JWST. For example, a spectrum for the most distant star Earendel, has also recently been taken and will probably be published soon. In future observation campaigns, we can systematically follow up NIRCam-detected lensed stars, if they are persistent sources, with NIRSpec spectroscopy in order to derive their properties.
“This study is also based on relatively short JWST exposure times of ~2h, whereas JWST is perfectly capable of reaching much higher signal-to-noise ratios through longer exposure times which means that future NIRSpec observations might well be able to detect absorption features in lensed stars at least in the brightest ones. Note that this would also be a compelling science case for the upcoming 30m-class telescopes like ESOs ELT, which will be able to reach similar sensitivities and resolutions as JWST, though be it at slightly lower wavelengths.Furtak
The preprint of their paper recently appeared online and is being reviewed for publication in the Monthly Notices of the Royal Astronomical Society.