HIP 56948 (HD 101364)—an 8.7 magnitude star in Draco—is more like our Sun than any other star yet discovered. It is 194 light years away and located at α2000 = 11h 40m 28s and δ2000 = +69° 00′ 31″, near Gianfar (λ Draconis) and the Draco-Ursa Major border, above the Big Dipper’s bowl.
With the exception of lithium, the elemental abundances are identical to that found in the Sun, within the observational uncertainties. As expected, lithium is severely depleted in HIP 56948, but not as much as in the Sun. This is to be expected for a solar twin about 1 Gyr younger than the Sun.
The temperature, luminosity, mass, and rotation of HIP 56948 almost exactly match that of the Sun. For example, HIP 56948 is only 17 ± 7 K hotter than the Sun, and its mass is 1.02 ± 0.02 M☉. Given all these similarities, it appears its most recently determined (1993) spectral type of G5 is incorrect. Or is it the spectral type of our Sun that is wrong (G2V)? Actually, it is quite difficult to make measurements of our Sun “as a star” because it is so incredibly close and bright.
HIP 56948 harbors no giant planets or “hot Jupiters” within or interior to its habitable zone, so there remains the enticing possibility that it may host a planetary system similar to our own, though no planets have yet been detected.
Incidentally, the next time you’ve got a good view of the Head of Draco and the “box” of Cepheus, cast your eyes toward a point halfway between the two. You’re looking towards where the rotational axis of the Sun points north. Like HIP 56948, it’s in Draco.
139 asteroids have orbits that lie entirely within Earth’s aphelion distance from the Sun (1.016725 AU). That number reduces to 50 inside Earth’s semimajor axis distance (1.000001 AU). That number further reduces to 26 inside Earth’s perihelion distance (0.983277 AU). Those 26 asteroids are listed below.
Only one asteroid lies entirely within Venus’s orbit (2020 AV2), and none are known inside Mercury’s orbit…so far. Asteroids inside of the Earth’s orbit are extremely difficult to detect since their angular distance from the Sun is never very large, and the glare of the Sun interferes. This is especially true for any asteroids that might exist inside of Mercury’s orbit.
An asteroid is given a provisional designation when it is discovered that begins with the year of discovery. After the orbit of the asteroid has been sufficiently well-determined, it is given a number. Then, eventually, the numbered asteroid is given a name.
Only 6 of the 26 asteroids entirely within Earth’s perihelion distance have received numbers, and only one of these has been given a name: Atira.
Interestingly, 12 of these 26 asteroids have been discovered since 2017, including 4 so far this year.
In the table below, i is the orbital inclination relative to the ecliptic plane, e is the orbital eccentricity, q is the perihelion distance, a is the semimajor axis distance, Q is the aphelion distance, and P is the orbital period. The table is listed in order of aphelion distance, smallest to largest.
IAU Minor Planet Center
We are the official body that deals with astrometric observations and orbits of minor planets (asteroids) and comets.
During the first half of 2021, I serendipitously captured six meteors on my telescope’s 17 x 11 arcminute video field of view while observing potential asteroid occultation events. I used the method described in There’s a Meteor in My Image to determine the radiant for each meteor. Here they are.
A sporadic meteor is any meteor that does not come from a known radiant.
If you have trouble seeing any of these meteors, you may want to use the full-screen button at the lower-right-hand corner of each video.
The Earth passed through an unexpected filament from Comet 109P/Swift-Tuttle, causing a spectacular enhancement of the Perseids on Saturday morning August 14 beginning around 0700 UT and continuing at least until 0945 UT when morning twilight began interfering with our observations. This is some 35 hours after the traditional peak (filament was at solar longitude ~141.5˚, whereas the traditional peak is at 140.0˚ – 140.1˚). Paul Martsching and I were observing NE of Ames, Iowa and saw single-observer observed rates of 40 to 60+ meteors per hour for an extended period. Many were bright (0th and 1st magnitude, some brighter). Paul’s peak hourly rate was 64 Perseids during the hour 0845 – 0945 UT.
The dip in the meteor counts around 0830 looks to be real, and appears to be corroborated by the radio meteor counts from Germany (shown at the top of this article). This could be due to a dip in the brighter meteor rate (but not the fainter ones we couldn’t see), or perhaps it was a dip in the overall rate as the Earth passed through two “strands” of the meteoroid filament.
“CBET 5016 (Jenniskens, 2021) states the peak was reached on Aug. 14, 08h02m UT (solar longitude 141.474 ± 0.005 degrees (equinox J2000.0)), with maximum ZHR between 130 ± 20 (calculated from CAMS Texas and California networks) and 210 ± 20 (calculated by K. Miskotte (DMS) from Pierre Martin’s visual observations) in good agreement with values calculated by H. Ogawa of the International Project for Radio Meteor Observation from radio forward scatter meteor observations. According to Peter Jenniskens (MeteorNews (b)), this probable filament may have been crossed over the last years, especially in 2018 (ZHR ~ 25 at solar longitude 140.95°) and 2019 (ZHR ~ 30 at solar longitude 141.02°) .”
Paul Martsching kept a detailed visual record of the outburst. He writes, “Apparently the ZHR was around double what we actually saw. The brightness index indicates a lot of faint meteors.”
Paul writes, “The rate went up to ~ 60/hour for nearly an hour; then fell back to ~ 40/hour for 45 minutes; then went back up to ~75/hour for 45 minutes; then seemed to be declining as morning twilight was interfering.”
Paul’s detailed log sheets are shown at the end of this article.
Meteor outbursts like this are rare, but they do occur from time to time. In the future, it would be nice if some of the automated meteor camera systems around the world could do some real-time processing in order to immediately alert visual observers of any outburst in progress, similar to what has often been done for auroral displays
Paul uses a talking clock and a steno pad to record the details of the meteors he sees, observing conditions, etc., without taking his eyes off the sky or needing to use a flashlight. He rolls a rubber band down the page to act as a guide for the pencil.
I have used a digital tape recorder with an external microphone that can be turned on and off for each event, and a talking clock. Unfortunately, I lost all that equipment in the Houston Memorial Day Weekend flood in 2015.
I am looking for a digital voice recorder that records the time each activation of the external microphone occurs. In other words, when I later play back each meteor description audio “snippet”, I want to be able to know exactly what the time was when the audio was recorded, thus eliminating the need for a talking clock. Does any such device exist?
A number of automated meteor cameras captured this outburst, but nothing can compare with seeing it visually under excellent conditions! I hope many others saw this event, but I suspect most visual observers did not go out, since it was after the predicted peak nights of Aug 11/12 and 12/13. A nice surprise, and on a weekend, too!
Do stars made of antimatter exist in the universe? Possibly.
One of the great mysteries of cosmology and astrophysics is that even though equal quantities of matter and antimatter appear to have been produced during the “Big Bang”, today there is only a negligible quantity of antimatter in the observable universe. We do not appear to live in a matter-antimatter symmetric universe.
If antimatter stars, “antistars”, do exist, how could we distinguish them from stars made of normal matter? The light emitted from an antistar would look identical to the light emitted by a normal-matter star.
But if normal matter were infalling upon an antistar, the contact between matter and antimatter would generate an annihilation spectrum of gamma ray photons that peaks around energy 70 MeV (half the mass of a neutral pion) up to a sharp cutoff around 938 MeV (mass of the proton).
A recent analysis of data collected by the Fermi Gamma-ray Space Telescope found fourteen possible antistars. These fourteen point sources produce a gamma-ray signature indicative of matter-antimatter annihilation. These point sources do not exhibit the characteristics of other known gamma-ray sources. For example, they are not, ostensibly, pulsars, active galactic nuclei, or black holes.
The positional error ellipses for these fourteen point sources range from 11×10 arcminutes up to 128×68 arcminutes (95% confidence). Here are optical images of these sources from the Palomar Digital Sky Survey, in order of right ascension (epoch 2000 coordinates).
Since there appears to be no known way to distinguish a star made of antimatter from one made of matter—except for the gamma-ray signature of matter infalling onto the antimatter star, a higher-resolution gamma-ray telescope or interferometer (10 – 1000 MeV) needs to be developed to localize these candidate sources to within a few arcseconds. Higher spectral resolution will help as well, allowing a more detailed characterization of the gamma-ray spectrum.
The highest-energy gamma ray photon ever recorded was recently observed by the Large High Altitude Air Shower Observatory (LHAASO) on Haizi Mountain, Sichuan province, China, during its first year of operation.
1.42 ± 0.13 PeV
That is 1.4 petaelectronvolts = 1.4 × 1015 eV! The origin of this fantastically energetic photon hasn’t been localized, but possible candidates are the Cygnus OB2 young massive cluster (YMC), the pulsar PSR 2032+4127, or the supernova remnant candidate SNR G79.8+1.2.
How much energy is 1.4 PeV, actually?
We can calculate the frequency of this photon using
where h = Planck’s constant = 4.135667696 × 10-15 eV·Hz-1 ν = the photon’s frequency E = the photon’s energy
Solving for ν, we get
ν = 3.4 × 1029 Hz
Next, we’ll calculate the photon’s wavelength using
where c = the speed of light = 299792458 m·s-1 λ = the photon’s wavelength
Solving for λ, we get
λ = 8.9 × 10-22 m
To give you an idea of just how tiny 8.9 × 10-22 meters is, the proton charge radius is 0.842 × 10-15 m, so 1.9 million wavelengths of this gamma ray photon would fit inside a single proton! An electron has an upper limit on its radius—if it can be said to have a radius at all—between 10-22 and 10-18 m. So between 1 and 2000 wavelengths of this gamma ray photon would fit inside a single electron.
Using Einstein’s famous equation E = mc2 we can find that each eV has a mass equivalent of 1.78266192 × 10-36 kg. 1.4 PeV then gives us a mass of 2.5 × 10-21 kg. That may not sound like a lot, but it is 1.5 million AMUs (Daltons), or a mass comparable to a giant molecule (a protein, for example) containing ~200,000 atoms.
This and other extremely high energy gamma ray photons are not directly detected from the Earth’s surface. The LHAASO detector array in China at 14,500 ft. elevation detects the air shower produced when a gamma ray (or cosmic ray particle) hits an air molecule in the upper atmosphere, causing a cascade of subatomic particles and lower-energy photons, some of which reach the surface of the Earth. It is the Cherenkov photons produced by the air shower secondary charged particles that LHAASO collects.
A giant cloud of mostly hydrogen gas with enough material to make over a million suns is heading towards our Milky Way at a speed of 45 miles per second. Called the Smith Cloud (after Gail Bieger-Smith who discovered it in 1963), this 9,800 × 3,300 ly high velocity cloud (HVC) is about 40,000 ly distant and is expected to slam into our Milky Way galaxy in about 27 million years, causing the birth of many new stars a quarter-way round the galaxy from us.
The Smith Cloud is located in the constellation Aquila, and has an apparent diameter around 11° across its long axis. It is only visible using radio telescopes (spin-flip transition ofneutral atomic hydrogen), or by detecting hydrogen absorption lines Doppler shifted and superimposed upon the spectra of more distant stars that are shining through the cloud.
The origin of the Smith Cloud is unknown. It may have originated within the Milky Way galaxy itself, or it may be extragalactic. The upcoming collision may not be the first time the Smith Cloud has encountered the disc of the Milky Way. It may be embedded in a large halo of dark matter which would have kept the cloud from being completely disrupted during any past encounters.
The Smith Cloud is a great example of an object that would never have been discovered were it not for radio astronomy. Felix J. Lockman, who has published extensively on the Smith Cloud, has created Radio Astronomy: Observing the Invisible Universe for The Great Courses. Dr. Lockman’s engaging lecture style, his clear explanations, and thorough knowledge of the subject matter makes this the perfect introduction to the subject. Highly recommended!
Incidentally, Jay Lockman discovered a region in Ursa Major that is relatively free of neutral hydrogen gas and dust, thus affording a clearer view into the distant universe. It is named, appropriately, the Lockman Hole.
Alig, C. et al. “Simulating the Impact of the Smith Cloud.” The Astrophysical Journal 869 (2018): 1-6. arXiv:1901.01639 [astro-ph.GA]
Hu, Y. et al. “Magnetic field morphology in interstellar clouds with the velocity gradient technique.” Nature Astronomy (2019): 1-7. arXiv:2002.09948 [astro-ph.GA]
Lockman, F.. “Accretion Onto the Milky Way: The Smith Cloud.” Proceedings of the International Astronomical Union 11 (2015): 9 – 12. arXiv:1511.05423 [astro-ph.GA]
The elements that make up the stars also exist here on Earth. In fact, our Earth, and indeed all the planets, were created from the dust and gas produced by previous generations of stars that existed before our Sun and solar system formed. We truly are made of stardust!
Stars are made up almost entirely of hydrogen and helium. Here is a table of the most abundant elements in our Sun.
% by atoms
Most abundant elements in the Sun
It is not a trivial matter to determine the abundance of elements in the Sun. For most elements, astronomers have to look at the strength of spectral absorption lines in the photosphere. Some elements, like fluorine, chlorine, and thallium, require looking for their spectral lines inside of sunspots, which are cooler-than-average regions of the photosphere. Other elements require that we look at spectral lines in the solar corona, or capture and analyze the solar wind. And some elements we are simply unable to detect.
The region of the photosphere that is amenable to spectral study represents only about 2% of the mass of the Sun. Since the Sun’s formation 4.6 Gyr ago, some gravitational settling of heavier elements and diffusion of hydrogen towards the surface means the Sun is not uniform in composition. Fortunately, the relative abundances of the elements heavier than helium are probably similar throughout the Sun.
Lithium, the third element in the periodic table after hydrogen and helium, is the odd element out. It has a relative abundance in the solar photosphere that is only 1/170th that found in meteorites. The Sun’s original supply of lithium has largely been destroyed by the high temperatures inside the pre-main-sequence Sun, and today at the hot bottom of the Sun’s convection zone.
Light pollution is a problem here on Earth, but on the Sun we have a problem with “line pollution”. There are so many spectral lines that the weak signatures from some elements become difficult or impossible to isolate and measure. There is much blending of overlapping lines, and some elements—most notably iron which is the ninth most abundant element in the Sun—are “superpolluters” with hundreds to thousands of spectral lines from both excited and ionized states.
Sometimes, the spectral lines of interest are in a region of the electromagnetic spectrum (ultraviolet, for example) that can only be observed from space, and that creates additional challenges.
Notably, the noble gases helium, neon, argon, krypton, and xenon have no photospheric absorption lines that can be observed, and we must look to coronal sources such as the solar wind, solar flares, or solar energetic particles for information about their abundances.
Helium—the second most abundant element in the Sun—requires an indirect approach combining a theoretical solar model and observational helioseismology data to tease out its abundance.
The following elements are undetectable in the Sun: arsenic, selenium, bromine, technetium, tellurium, iodine, cesium, promethium, tantalum, rhenium, mercury, bismuth, polonium, astatine, radon, francium, radium, actinium, protactinium, and all the synthetic elements above uranium on the period table.
Interestingly, helium was discovered in the Sun before it was discovered on Earth! That’s why this element is name after Helios, the Greek god of the Sun.
The energy source that allows stars to shine steadily, often for billions of years, is fusion. Fusion in a star can only occur where both the temperature and pressure are very high. Usually (but not always!), this occurs in the core of the star. When the element hydrogen fuses into helium, a huge amount of energy is released in the process. Lucky for us, fusing hydrogen into helium is difficult to do in a one-solar-mass star. On average, any particular hydrogen atom in our Sun has to “wait” about five billion years before having the “opportunity” to participate in a fusion reaction!
In order for sustained fusion to occur in the core of a star, the star must have sufficient mass so that the core temperature and pressure is high enough. Present thinking is that the lowest mass stars where sustained fusion can occur have about 75 times the mass of Jupiter, or about 7% the mass of the Sun.
As the expanding universe cooled, the first neutral1 hydrogen atoms formed about 380,000 years after the Big Bang (ABB), and most of the hydrogen in the universe remained neutral until the first stars began forming at least 65 million years ABB.
The period of time from 380,000 to 65 million years or so ABB is referred to as the “dark ages” since at the beginning of this period the cosmic background radiation from the Big Bang had redshifted from visible light to infrared so the universe was truly dark (in visible light) until the first stars began to form at the end of this period.
All the while, neutral hydrogen atoms occasionally undergo a “spin-flip” transition where the electron transitions from the higher-energy hyperfine level of the ground state to the lower-energy hyperfine level, and a microwave photon of wavelength 21.1061140542 cm and frequency 1420.4057517667 MHz is emitted.
Throughout the dark ages, the 21 cm emission line was being emitted by the abundant neutral hydrogen throughout the universe, but as the universe continued to expand the amount of cosmological redshift between the time of emission and the present day has been constantly changing. The longer ago the 21 cm emission occurred, the greater the redshift to longer wavelengths. We thus have a great way to map the universe during this entire epoch by looking at the “spectrum” of redshifts of this particular spectral line.
380,000 and 65 million years ABB correspond to a cosmological redshift (z) of 1,081 and 40, respectively. We can calculate what the observed wavelength and frequency of the 21 cm line would be for the beginning and end of the dark ages.
The observed wavelength (λobs) for the 21 cm line (λemit) at redshift (z) of 1,081 using the above equation gives us 22,836.8 cm or 228.4 meters.
That gives us a frequency (ν) of 1.3 MHz (using the equation above), where the speed of light c = 299,792,458 meters per second.
So a 21 cm line emitted 380,000 years ABB will be observed to have a wavelength of 228.4 m and a frequency of 1.3 MHz.
Using the same equations, we find that a 21 cm line emitted 65 Myr ABB will be observed to have a wavelength of 8.7 m and a frequency of 34.7 MHz.
We thus will be quite interested in taking a detailed look at radio waves in the entire frequency range 1.3 – 34.7 MHz, with corresponding wavelengths from 228.4 m down to 8.7 m.2
The interference from the Earth’s ionosphere and the ever-increasing cacophony of humanity’s radio transmissions makes observing these faint radio signals all but impossible from anywhere on or near the Earth. Radio astronomers and observational cosmologists are planning to locate radio telescopes on the far side of the Moon—both on the surface and in orbit above it—where the entire mass of the Moon will effectively block all terrestrial radio interference. There we will finally hear the radio whispers of matter before the first stars formed.
1 By “neutral” we mean hydrogen atoms where the electron has not been ionized and resides in the ground state—not an excited state.
2 Incidentally, the 2.7 K cosmic microwave background radiation which is the “afterglow” of the Big Bang itself at the beginning of the dark ages (380,000 years ABB), peaks at a frequency between 160 and 280 GHz and a wavelength around 1 – 2 mm. So this is a much higher frequency and shorter wavelength than the redshifted 21 cm emissions we are proposing to observe here.
Ananthaswamy, Anil, “The View from the Far Side of the Moon”, Scientific American, April 2021, pp. 60-63
During the second half of 2020, I serendipitously captured six meteors on my telescope’s 17 x 11 arcminute video field of view while observing potential asteroid occultation events. I used the method described in There’s a Meteor in My Image to determine the radiant for each meteor. Here they are.
A sporadic meteor is any meteor that does not come from a known radiant.
None of these meteors were particularly bright, unfortunately, so you may want to use the full-screen button at the lower-right-hand corner of each video to see them well.