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ATLANTIC SKIES: How bright do the stars shine? The magnitude system explained –



Some of my readers have queried me as to why the brighter objects in the night sky have negative magnitude values, while the fainter ones have positive values, when, logically (at least to them), it should be the other way around.

For this seemingly “backward” rating system, we can thank the ancient Greek astronomer Hipparchus, who, in 129 BC, drew up the first recognized star chart. On this chart, he listed the magnitude (from Latin magnitudo or magnus meaning “great”) of the stars he could see in the night sky. Hipparchus listed the brightest stars that he could see with his naked eye as magnitude +1.0 stars, those half as bright as the magnitude +1.0 stars as magnitude 2.0 stars, and so on, until reaching magnitude +6.0, the faintest he could see.

His magnitude scale remained in use for rating the brightness of the stars (and other celestial objects by comparison) for the next 1,400 years. It wasn’t until 1609, when Italian astronomer Galileo (1564-1642) developed his first telescope and observed much fainter stars than those listed on the star charts in use at that time, that the magnitude scale was extended (with ascending positive numbers) to include the fainter stars.

In the mid-1850s, when astronomers discovered that some magnitude +1.0 stars are brighter than others, the scale was again extended outward, this time with ascending negative values to reflect the brighter stars.

The stars Rigel (Orion), Capella (Auriga), Arcturus (Bootes), and Vega (Lyra) were listed at magnitude 0.0, while stars brighter than these were given negative values. Sirius, the brightest star in the night sky, is rated at magnitude -1.43 , while our sun is rated at magnitude -26.7.

Planets and other celestial objects can also be rated on the magnitude scale. Venus, at its brightest, shines at magnitude -4.4, while the full moon beams (on average) at magnitude -12.6.

The faintest stars that the average human, naked-eye can see (under a clear sky from a dark site) is magnitude +6.0, while binoculars can boost that to magnitude +10. In contrast, the Hubble Space Telescope can see stars as faint as magnitude +30.

With stronger telescopes, the magnitude scale for stars was again adjusted.


What does it mean?

A star’s apparent brightness or luminosity refers to the amount of light energy (from thermonuclear fusion within the star’s core) it emits, and how much of that energy passes per second through a square meter of the star’s surface area. Basically, how bright a star appears depends on how much of its light energy per second strikes the area of a light detector (in our case, the human eye). The apparent brightness we see or measure is inversely proportional to the square of our distance from the star, with the apparent brightness diminishing as the distance squares.

Astronomers use the terms “apparent magnitude” and “absolute magnitude” when denoting a star’s brightness. Apparent magnitude is how bright the star appears to an Earth-bound observer, and is directly related to a star’s apparent brightness.

Stellar measurements in the 19th century indicated that magnitude +1.0 stars are approximately 100 times brighter than magnitude +6.0 stars (i.e., it would take 100 magnitude +6.0 stars to provide as much light as a single magnitude +1.0 star). Subsequently, the stellar magnitude scale was modified so that a magnitude difference of five corresponded exactly to a factor of 100 times difference in brightness., while a difference of one magnitude equaled a difference factor of 2.512 in brightness.

This resulting stellar magnitude rating system was based on a logarithmic scale, with whole numbers, and fractions thereof, indicating varying ratios of brightness (e.g., 0 = 1 to 1; 0.2 = 1.2 to 1; 0.5 = 1.6 to 1; 1 = 2.5 to 1; 5 = 100 to 1, etc.). A star’s apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any dimness of the star’s light caused by the interference of interstellar dust along the line of sight of the observer.

When astronomers want to measure how intrinsically bright a star is regardless of its distance from Earth, they measure the star’s absolute magnitude, or its apparent magnitude if all the stars it is being compared to were placed at 10 parsecs distance from Earth. With one parsec equaling 3.26 light-years (a light-year is the distance light travels through the vacuum of space in one year; approximately 10 trillion kilometres), 10 parsecs equals 32.6 light-years, or approximately 100 trillion kms. A star’s absolute magnitude measures its true energy output (its luminosity).

As with the apparent magnitude scale, the absolute magnitude scale is also “backward”, giving less luminous stars ascending positive values, and more luminous stars ascending negative ones. For celestial objects such as comets and asteroids, the absolute magnitude scale (also with positive through negative values) is based on how bright the object would appear to an observer standing on the sun if the object were 1 AU (149,597,871 kms) away.

This week’s sky

Mercury (magnitude -0.8) is visible low (about eight degrees) above the northwest horizon shortly after 9 p.m., before dropping from view shortly after 10 p.m. This bright but small planet (heading towards its greatest eastern elongation from the sun on June 2) achieves an altitude of 18 degrees in the evening sky by May 31. It reaches its half-phase (called dichotomy) on May 29.

Venus (magnitude -4.3) appears only about 13 degrees above the western horizon shortly after 9 p.m., before setting shortly before 11 p.m.

Jupiter (magnitude -2.5) rises in the southeastern sky shortly before 1 a.m., reaching 22 degrees height in the southern sky before fading from view around 5:15 a.m.

Saturn (magnitude +0.48) follows Jupiter into the southeastern dawn sky around 1 a.m., rising to about 23 degrees above the southern horizon before it fades from sight shortly before 5 a.m.

Mars (magnitude +0.16) rises in the southeast around 2:30 a.m., reaching an altitude of about 20 degrees above the horizon before fading from view a few minutes before 5 a.m.

Currently at magnitude +4.5, Comet C/2020 F8 SWAN is now in the constellation of Perseus – the Warrior Prince. This fading comet will be difficult to see, as it reaches an altitude of only about 10 degrees above the northeastern horizon between 4 a.m. and 5 a.m., before the glow of the rising sun overtakes it. With clear skies and an unobstructed view of the northeastern horizon, it might still be seen in binoculars and small scopes.

Until next week, clear skies.


May 29 – Mercury reaches dichotomy

May 30 – First quarter moon

Glenn K. Roberts lives in Stratford, P.E.I., and has been an avid amateur astronomer since he was a small child. He welcomes comments from readers, and anyone who would like to do so is encouraged to email him at


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NASA Moves Forward With Next-Gen Solar Sail Project – ExtremeTech



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Getting from point A to point B in the solar system is no simple feat, and inefficient, heavy rockets aren’t always the best way. Therefore, NASA has announced it is moving ahead with a new solar sail concept that could make future spacecraft more efficient and maneuverable. The Diffractive Solar Sailing project is now entering phase III development under the NASA Innovative Advanced Concepts (NIAC) program, which could eventually lead to probes that use solar radiation to coast over the sun’s polar regions. 

The concept of solar sails is an old one — they were first proposed in the 1980s. The gist is that you equip a vessel with a lightweight sail that translates the pressure from solar radiation into propulsion. The problem is that a solar sail has to be much larger than the spacecraft it’s dragging along. Even a low-thrust solar sail would need to be almost a square kilometer, and you need to keep it intact over the course of a mission. Plus, you have little choice but to fly in the direction of sunlight, so you have to make tradeoffs for either power or navigation. Futuristic diffractive light sails could address these shortcomings. 

This work is being undertaken at the Johns Hopkins University Applied Physics Laboratory under the leadership of Amber Dubill and co-investigator Grover Swartzlander. The project progressed through phase I and II trials, which had the team developing concept and feasibility studies on diffractive light sails. The phase III award ensures $2 million in funding over the next two years to design and test the materials that could make diffractive light propulsion a reality. 

A standard lightsail developed by the Planetary Society in 2019.

A diffractive light sail, as the name implies, takes advantage of a property of light known as diffraction. When light passes through a small opening, it spreads out on the other side. This could be used to make a light sail more maneuverable so it doesn’t need to go wherever the solar winds blow. 

The team will design its prototypes with several possible mission applications in mind. This technology most likely won’t have an impact on missions to the outer solar system where sunlight is weaker and the monumental distances require faster modes of transportation. However, heliophysics is a great use case for diffractive lightsailing as it would allow visiting the polar regions of the sun, which are difficult to access with current technology.

A lightsail with the ability to essentially redirect thrust from a continuous stream of sunlight would be able to enter orbit over the poles. It may even be possible to maneuver a constellation of satellites into this difficult orbit to study the sun from a new angle. In a few years, NASA may be able to conduct a demonstration mission. Until then, it’s all theoretical.

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Tau Herculid meteor shower will happen Monday night, Tuesday morning – USA TODAY



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461 new objects discovered at the edge of our solar system

It increases our knowledge of what’s floating in the Kuiper Belt by a significant margin.

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  • The meteor shower, known as the tau Herculids, could be spectacular, or it could be a total dud.
  • If it does reach thousands of meteors per hour, it would be a “meteor storm.”
  • Maximum activity is expected around 1 a.m. EDT Tuesday morning, May 31.

Sky watchers could be in for a memorable spectacle Monday night and early Tuesday morning as the Earth passes through debris from a disintegrating comet, leading to a potential meteor shower with thousands of shooting stars per hour. 

The meteor shower, known as the tau Herculids, could be spectacular, or it could be a total dud, astronomers said.  

“This is going to be an all or nothing event,” NASA meteor expert Bill Cooke said in a statement. If it does reach thousands of meteors per hour, it would be a “meteor storm,” as opposed to a shower.

There is “a small chance of something extraordinary – perhaps one of the most dramatic meteor displays since the spectacular Leonid meteor showers of more than 20 years ago,” said Joe Rao of

Maximum activity is expected around 1 a.m. EDT Tuesday, the Space Weather Archive blog said. 

The comet is known as 73P/Schwassmann-Wachmann 3 (SW3), named after the two German astronomers who discovered it in 1930. The comet is breaking into dozens of pieces as it orbits the sun, which it does every 5.4 years, NASA said. 

EYE TO THE SKY: How to watch every meteor shower in 2022

In all, SW 3 has broken into more than 68 fragments. At its most recent appearance in March 2017, it showed signs that it sheds pieces in each return through the inner solar system, Rao said. 

If it makes it to us this year, the debris from the comet will strike Earth’s atmosphere at 10 miles per second, which is on the slow side for a good meteor shower. 

Stargazers will pay attention this year because meteors should be high in the night sky at the forecast peak time, NASA said. The higher the radiant point is in the sky, the more meteors you are likely to see.

Even better, the moon is new, so there will be no moonlight to wash out the faint meteors.

For ideal viewing of this or any meteor shower, find a spot away from city lights. Your eyes will need to adjust to the darkness, which could take 15 to 20 minutes. Watching meteor showers can take time, so be patient, experts advise. It could be worth the wait!

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Toward customizable timber, grown in a lab – EurekAlert



image: In an effort to provide an environmentally friendly and low-waste alternative, researchers at MIT have pioneered a tunable technique to generate wood-like plant material in a lab.
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Credit: Image courtesy of Luis Fernando Velásquez-García, Ashley Beckwith, et al

Each year, the world loses about 10 million hectares of forest — an area about the size of Iceland — because of deforestation. At that rate, some scientists predict the world’s forests could disappear in 100 to 200 years.

In an effort to provide an environmentally friendly and low-waste alternative, researchers at MIT have pioneered a tunable technique to generate wood-like plant material in a lab, which could enable someone to “grow” a wooden product like a table without needing to cut down trees, process lumber, etc.

These researchers have now demonstrated that, by adjusting certain chemicals used during the growth process, they can precisely control the physical and mechanical properties of the resulting plant material, such as its stiffness and density.

They also show that, using 3D bioprinting techniques, they can grow plant material in shapes, sizes, and forms that are not found in nature and that can’t be easily produced using traditional agricultural methods.

“The idea is that you can grow these plant materials in exactly the shape that you need, so you don’t need to do any subtractive manufacturing after the fact, which reduces the amount of energy and waste. There is a lot of potential to expand this and grow three-dimensional structures,” says lead author Ashley Beckwith, a recent PhD graduate.

Though still in its early days, this research demonstrates that lab-grown plant materials can be tuned to have specific characteristics, which could someday enable researchers to grow wood products with the exact features needed for a particular application, like high strength to support the walls of a house or certain thermal properties to more efficiently heat a room, explains senior author Luis Fernando Velásquez-García, a principal scientist in MIT’s Microsystems Technology Laboratories.

Joining Beckwith and Velásquez-García on the paper is Jeffrey Borenstein, a biomedical engineer and group leader at the Charles Stark Draper Laboratory. The research is published today in Materials Today.

Planting cells

To begin the process of growing plant material in the lab, the researchers first isolate cells from the leaves of young Zinnia elegans plants. The cells are cultured in liquid medium for two days, then transferred to a gel-based medium, which contains nutrients and two different hormones.

Adjusting the hormone levels at this stage in the process enables researchers to tune the physical and mechanical properties of the plant cells that grow in that nutrient-rich broth.

“In the human body, you have hormones that determine how your cells develop and how certain traits emerge. In the same way, by changing the hormone concentrations in the nutrient broth, the plant cells respond differently. Just by manipulating these tiny chemical quantities, we can elicit pretty dramatic changes in terms of the physical outcomes,” Beckwith says.

In a way, these growing plant cells behave almost like stem cells — researchers can give them cues to tell them what to become, Velásquez-García adds.

They use a 3D printer to extrude the cell culture gel solution into a specific structure in a petri dish, and let it incubate in the dark for three months. Even with this incubation period, the researchers’ process is about two orders of magnitude faster than the time it takes for a tree to grow to maturity, Velásquez-García says.

Following incubation, the resulting cell-based material is dehydrated, and then the researchers evaluate its properties.

Wood-like characteristics

They found that lower hormone levels yielded plant materials with more rounded, open cells that have lower density, while higher hormone levels led to the growth of plant materials with smaller, denser cell structures. Higher hormone levels also yielded plant material that was stiffer; the researchers were able to grow plant material with a storage modulus (stiffness) similar to that of some natural woods.

Another goal of this work is to study what is known as lignification in these lab-grown plant materials. Lignin is a polymer that is deposited in the cell walls of plants which makes them rigid and woody. They found that higher hormone levels in the growth medium causes more lignification, which would lead to plant material with more wood-like properties.

The researchers also demonstrated that, using a 3D bioprinting process, the plant material can be grown in a custom shape and size. Rather than using a mold, the process involves the use of a customizable computer-aided design file that is fed to a 3D bioprinter, which deposits the cell gel culture into a specific shape. For instance, they were able to grow plant material in the shape of a tiny evergreen tree.

Research of this kind is relatively new, Borenstein says.

“This work demonstrates the power that a technology at the interface between engineering and biology can bring to bear on an environmental challenge, leveraging advances originally developed for health care applications,” he adds.

The researchers also show that the cell cultures can survive and continue to grow for months after printing, and that using a thicker gel to produce thicker plant material structures does not impact the survival rate of the lab-grown cells.

“Amenable to customization”

“I think the real opportunity here is to be optimal with what you use and how you use it. If you want to create an object that is going to serve some purpose, there are mechanical expectations to consider. This process is really amenable to customization,” Velásquez-García says.

Now that they have demonstrated the effective tunability of this technique, the researchers want to continue experimenting so they can better understand and control cellular development. They also want to explore how other chemical and genetic factors can direct the growth of the cells.

They hope to evaluate how their method could be transferred to a new species. Zinnia plants don’t produce wood, but if this method were used to make a commercially important tree species, like pine, the process would need to be tailored to that species, Velásquez-García says.  

Ultimately, he is hopeful this work can help to motivate other groups to dive into this area of research to help reduce deforestation.

“Trees and forests are an amazing tool for helping us manage climate change, so being as strategic as we can with these resources will be a societal necessity going forward,” Beckwith adds.

This research is funded, in part, by the Draper Scholars Program.


Written by Adam Zewe, MIT News Office

Additional background

Paper: “Physical, mechanical, and microstructural characterization of novel, 3D-printable, tunable, lab-grown plant materials generated from Zinnia elegans cell cultures”

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