Category Archives: Stars

Astronomy, Nebula, Physics, Space Travel, Stars

At what speed does the interstellar medium become lethal to high speed flight?

The dilute interstellar medium permeates space at a density of about one hydrogen atom per cubic centimeter. This image shows an all-sky map of this hydrogen observed by the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey (Haffner, L. M. et al, 2003, Astrophysical Journal Supplement, 149, 405).. The Wisconsin H-Alpha Mapper is funded by the (US) National Science Foundation.

We do not really know what the interstellar medium looks like at the human-scale. If it is just stray hydrogen atoms you will just experience a head-on flow of ‘cosmic rays’ that will collide with your spacecraft and probably generate secondary radiation in the skin of your ship. This can be annoying, but it can be shielded so long as the particles are not ultra-relativistic. At spacecraft speeds of 50-90% the speed of light, these particles are not likely to be a real problem. At speeds just below the speed of light, the particles are ultra-relativistic and would generate a very large x-ray and gamma-ray background in the skin of your ship.

As it turns out, our solar system is inside a region called the Local Bubble where the density of hydrogen atoms is about 100 times lower that in the general interstellar medium. This Bubble, produced by an ancient supernova, extends about 300 light years from the Sun but has an irregular shape. There are thousands of stars within this region which is enough to keep us very busy exploring safely. Here is one version of this region by astronomers at the Harvard-Smithsonian Center for Astrophysics.

Interstellar space also contains a few microscopic dust grains (micron-sized is common) in a region about a few meters on a side. At their expected densities you are probably in for a rough ride, but it really depends on your speed. The space shuttle, encountering flecks of paint traveling at 28,000 mph (about 6 miles/second or 0.005 percent the speed of light) is pitted and pierced by these fast moving particles, but dust grains have masses a thousand times smaller than the smallest paint fleck, so at 0.005 percent light speed, they will not be a problem.

At 50 percent the speed of light which is the minimum for interstellar travel you will cover enough distance in a short amount of time, that your likelihood of encountering a large interstellar dust grain becomes significant. Only one such impact would be enough to cause severe spacecraft damage given the kinetic energy involved.

A large dust grain might have a mass of a few milligrams. Traveling at 50% the speed of light, its kinetic energy is given non-relativistically by 1/2 mv^2 so E = .5 (0.001 grams) x (0.5 x 3 x 10^10 cm/sec) = 1.1 x 10^17 ergs. This, equals the kinetic energy of a 10 gram bullet traveling at a speed of 1500 kilometers per second, or the energy of a 100 pound person traveling at 13 miles per second! The point is that at these speeds, even a dust grain would explode like a pinpoint bomb, forming an intense fireball that would melt through the skin like a hot poker melts a block of cheese.

The dust grains at interstellar speeds become lethal interstellar ‘BB shots’ pummeling your spacecraft like rain. They puncture your ship, exploding in a brief fireball at the instant of contact.

Your likelihood of encountering a deadly dust grain is simply dependent on the volume of space your spacecraft sweeps out. The speed at which you do this only determines how often you will encounter the dust grain in your journey. At 10,000 times the space shuttle’s speed, the collision vaporizes the particles and a fair depth of the spacecraft bulkhead along the path of travel.

But the situation could well be worse than this if the interstellar medium contains lots of ice globules from ancient comets and other things we cannot begin to detect in interstellar space. These impacts even at 0.1c would be fatal…we just don’t know what the ‘size spectrum’ of matter is between interstellar ‘micron-sized’ dust grains, and small stars, in interstellar space.

My gut feeling is that interstellar space is rather filthy, and this would make interstellar, relativistic travel, not only technically difficult but impossible to boot! Safe speeds for current technology would be only slightly higher than space shuttle speeds especially if interstellar space contains chunks of comet ice.

This is an issue that no one in the science fiction world has even bothered to explore! The only possible exception is in Star Trek where the Enterprise is equipped with a forward-directed ‘Brussard Deflector’ (that big blue dish just below the main saucer) which is supposed to sweep away particles before they arrive at the ship. This is very dubious technology because hydrogen atoms are not the main problems a ship like that would have to worry about, especially traveling inside a planetary system at sub-light speeds. It’s dust grains!

Astronomy, Nebula, Stars

Why doesn’t the Sun blow up?

In fact, the Sun is doing a slow-motion explosion. It is shedding about 600 million tons every second in light energy, and it is loosing about 100 trillionth of its mass every year in the so-called solar wind. Here is a satellite photo of one of these mass ejections seen by the NASA/ESA SOHO satellite on December 2, 2003. These are dramatic events and often eject ‘a billion tons’ of plasma every few weeks or months. As impressive as they are, the sun is far more massive by a factor of a billion-billion times (1018).

But the sun will never blow up the way we think of a genuine explosion. It is the wrong kind of star to be either a nova or a supernova. It has no companion star for mass-transfer, and its mass is well below the 6-8 solar-mass limit when supernova detonations start to occur.

The energy of the Sun, the thermonuclear fusion which produces all the heat and light, is occurring in the core of the Sun. The weight of all the mass in the Sun in the overlying layers is so enormous that the Sun is in an equilibrium state where the internal thermal pressure is balanced by the gravitational pressure directed inwards.

Eventually, this balance will cease as the core depletes its hydrogen fuel. The core will collapse and heat up causing the outer layers to expand as a planetary nebula like the one shown here: NGC 6720 (Credit:ESA). This is still not a detonation that shatters the sun into interstellar space. In fact, more than 90% of its mass is left behind as a white dwarf ,which is a stable configuration of matter.

Astronomy, Stars

What is a red giant star?

This is the first direct image of a star other than the Sun, made with the Hubble Space Telescope. Called Alpha Orionis, or Betelgeuse, it is a red supergiant star marking the shoulder of the winter constellation Orion the Hunter. The Hubble image reveals a huge ultraviolet atmosphere with a mysterious hot spot on the stellar behemoth’s surface. The enormous bright spot, which is many hundreds times the diameter of Sun, is at least 2, 000 Kelvin degrees hotter than the surface of the star.

A red giant star is a star with a mass like our Sun that is in the last phase of its life. Hydrogen fusion reactions have become less efficient in the core region, and with gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core. This increases the luminosity of the star enormously (up to 1000 times the Sun) and it expands. The outer layers then cool to only 3000 K or so and you get a red star, but its size is now equal to the orbit of Mercury or Venus…or even the Earth! After a few more millions of years, the star evolves into a white dwarf-planetary nebula system and then it’s all over for the star.

The closest red giant star to our sun is Gamma Crucis (also referred to as Gacrux). It is the third-brightest star in the Southern Cross. Unlike its blue-white neighbors in the constellation, Gacrux is a bright red giant. Gacrux is also considered the nearest red giant to Earth, at a distance of roughly 88 light years.

Astronomy, Stars

Can you see stars from the bottom of a well?

From the bottom of a 6-foot diameter well, at a depth of say 50 feet, the sky would subtend the same angle to the eye as a 1.5 inch diameter tube of length 1 foot held up to the eye. This is about the same size as the paper tube in American ‘kitchen towel’ paper. If you do this experiment in the daytime you will see that no stars come out by sighting through such a tube. So, by direct experiment with a similar geometry, the answer is no, you cannot see stars in the daytime at the bottom of a well.

As a caviat, Ken Tapping an astronomer at the NRC Herzberg Institute of Astrophysics notes: “If we were on the Moon’s surface, where there is no atmosphere, we could simply shade out the Sun and the reflected glare from the ground and see the stars perfectly well. On the Earth, our atmosphere tends to scatter the sunlight, which is what makes the sky look blue. This blue is sufficiently bright that it is very difficult to see the stars through it, although on really clear days, in dark places such as the bottom of a well, where reflected light from things on the ground isn’t reaching your eyes, it is sometimes possible to see a star or two.”

Another answer, is given by David Hughes in the Quarterly Journal of the Royal Astron. Soc., 1983, vol. 24, pp 246-257.

This mistaken notion was first mentioned by Aristotle and other ancient sources, and was widely assumed to be correct by many literary sources of the 19th century, and even believed by some astronomers. But every astronomer who has ever tested this by experiment came away convinced it was impossible.

Separate experiments to attempt to see Vega and Pollux through tall chimneys were performed by J. A. Hynek and A. N. Winsor. They were unable to detect the stars under near perfect conditions, even with binoculars. The daytime sky is simply too bright to allow us to see even the brightest stars (although Sirius can sometimes be glimpsed just after the Sun rises if you know exactly where to look.) Venus can be seen as a tiny white speck but again, you have to be looking exactly at the right spot. According to Starwaders.com

Venus is a tiny point of light during daylight and it cannot be seen if the eye is not focussed at the furthest distance, i.e. infinity. In between looking at what is apparently a blank blue sky, briefly flick your gaze every few seconds to an object on the horizon or even onto a tree a hundred meters away. This pulls the eye lens into long distance focus. You could have been looking directly at the point where Venus was and not seen a thing, but having nudged your eyes into infinity focus, be totally surprised to see the bright Venus diamond suddenly become “as clear as daylight”. You will wonder why you could not see it before and why others around you cannot see it.

The most likely explanation for the old legend is that stray bits of rubbish get caught in the updraft and catch the sunlight as they emerge from the chimney. It is possible to see stars in the daytime with a good telescope, as long as it has been prefocused and can be accurately pointed at a target.

Astronomy, Stars

What is the relationship between a star’s color and its temperature?

Color and temperature are related by the famous Planck, black body formula shown here in a NASA-Webb Space Telescope illustration. Actually, the formula gives the intensity of a black body at any wavelength given its temperature. The wavelength where the peak of this curve occurs is determined by the temperature of the black body. You then have to relate this peak wavelength of emission to how the human eye identifies color in the visual part of the electromagnetic spectrum. For example, the wavelength of peak emission is

                                      2897
Wavelength (micrometers)  =    ------------------
                                 Temperature (K)

so that for the Sun with a temperature of 5700 K, the peak of the black body curve occurs at 2897/5700 = 0.51 micrometers or 5100 Angstroms. A cool M-type star can have T = 2500 K so their peak emission occurs at 1.16 micrometers in the deep or ‘far’ red part of the spectrum.

Astronomy, Stars

How big is Proxima Centauri?

Proxima Centauri, shown hear in a Hubble image (Credit:NASA), is a dM5e star (dwarf M5 emission-line star) with a luminosity of 0.00006 times the sun that was discovered in 1915 by the Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, when it was at that time 0.1 light years closer to our sun than Alpha Centauri.

Because of Proxima Centauri’s proximity to Earth, its angular diameter can be measured directly and is 1.02 ± 0.08 milliarcsec. At its distance, that means it is about one-seventh the physical diameter of the Sun. By comparison, it is about 50% larger that Jupiter.

Today, its orbit around Alpha Centauri (distance = 4.395 light years) now puts it at a distance of 4.223 light years according to the Hipparcos Satellite.

Proxima is located about 13,000 AUs from Alpha Cantauri A and B. The star is located roughly a fifth of a light-year from the AB binary pair and, if gravitationally bound to it, may have an orbital period of around half a million years. According to Anosova et al (1994), however, its motion with respect to the AB pair is hyperbolic.

Alpha Centauri A and B orbit each other at a distance of about 2.2 billion miles (3.6 billion kilometers), a bit more than the distance from the Sun to planet Uranus. It takes 80 years for them to complete an orbit. Proxima Centauri is nearer to Earth than the other two stars, by the rather large distance — roughly 10,000 times the distance from Earth to the Sun. All three known stars in the system were born about 4.85 billion years ago, astronomers believe. Our Sun began shining about 4.6 billion years ago. The A and B stars are both about the same temperature as the Sun. Proxima Centauri is about seven times smaller than the Sun. It contains just enough mass to cause hydrogen to burn, and it is much cooler and, intrinsically, only about 1/150th as bright as the Sun. This small star is barely a star at all, in fact. Its mass is just above that of brown dwarfs, a class of object that seems to straddle the definition between stars and planets. Though 150 times more massive than Jupiter, Proxima Centauri is only about 1.5 times bigger than the planet.

On August 24, 2016, the European Southern Observatory announced the discovery of Proxima b, a planet orbiting the star at a distance of roughly 0.05 AU (7,500,000 km) and an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. The equilibrium temperature of Proxima b is estimated to be within the range where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri. Previous searches for orbiting companions had ruled out the presence of brown dwarfs and supermassive planets orbiting Proxima.

Astronomy, Physics, Stars

Why aren’t there any green stars?

The color of a star is a combination of two phenomena. The first is the star’s temperature. This determines the wavelength (frequency) where the peak of its electromagnetic radiation will emerge in the spectrum. A cool object, like an iron rod heated to 3000 degrees, will emit most of its light at wavelengths near 9000 Angstroms ( the far-red part of the visible spectrum) in wavelength. A very hot object at a temperature of 30,000 degrees will emit its light near a wavelength of 900 Angstroms (the far-ultraviolet part of the visible spectrum). The amount of energy emitted at other wavelengths is precisely determined by the bodies temperature, and by Planck’s radiation law of ‘black bodies’. (Credit: Wikipedia)

It shows that as the temperature of the object increases, the peak shifts further to short wavelengths. But the phenomenon we call ‘color’ is another matter. Color does not exist as an objective property of nature.

Color is a perception we humans have because of the kinds of pigments used in our retinae. Our eyes do not sense light evenly across the visible spectrum but have a greater sensitivity for green light, and somewhat less so for red and blue light as the response spectrum below illustrates:

In effect, what you have to do is ‘multiply’ the spectrum of light you receive from a heated body, by the response of the eye to the various wavelengths of light in the spectrum. When this happens, a very unusual thing happens.

If I were to figure out how hot a star would have to be so that the peak of its emission was in the ‘green’ area near 4000 Angstroms, I would estimate that the temperature of the star would have to be about 10,000 degrees. There are many such stars in the sky. The two brightest of these ‘A-type’ stars are Vega in the constellation Lyra, and Sirius in Canes Major. But if you were to look at them in the sky, they would appear WHITE not green! Stars are ranked according to increasing temperature by the sequence of letters:

Type..... Color...............Temperature

O........ blue................30,000
B........ blue-white..........20,000
A........ white...............10,000
F........ yellow-white........ 8,000
G........ yellow.............. 5,000
K........ orange.............. 4,000
M........ red..................3,000

This is NOT the same sequence of colors you see in a rainbow (red, orange, yellow, green, blue, indigo, violet) because the distribution of energy in the light source is different, and in the case of the rainbow, optical refraction in a raindrop is added.

Another factor working against us is that we see stars in the sky using our black/white rods not our color-sensitive cones. This means that only the very brightest stars have much of a color, usually red, orange, yellow and blue. By chance there are no stars nearby that would have produced green colors had their spectral shapee been just right.

So, there are no genuinely green stars because stars with the expected temperature emit their light in a way that our eye combines into the perception of ‘whiteness’.

For more information on star colors, have a look at the article by Philip Steffey in the September, 1992 issue of Sky and Telescope (p. 266), which gives a thorough discussion of stellar colors and how we perceive them.

Astronomy, Black Hole, Stars

Is there a minimum and a maximum size to stars and black holes?

Stars span an enormous range of sizes as the figure above shows. Credit:Wikipedia-Dave Jarvis:

The minimum size for a star is believed to be near 0.04 times the mass of the Sun or about 80 times the mass of Jupiter. An object called a brown dwarf is really a large planet which was not massive enough for thermonuclear fusion to get ignited in the core. The difference between a brown dwarf and a planet is believed to be about 13 times the mass of Jupiter. The closest red dwarf is Proxima Centauri with a masss of about 130 times Jupiter. The closest brown dwarf to our sun as of 2014 is about 7.5 light years away and is called WISE J085510.83-071442.5, and is now the record-holder for the coldest brown dwarf, with a temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius) and a mass of about 3-10 times Jupiter. The exciting thing about red dwarf stars is that they burn their nuclear fuel so slowly that they exist as stars for 10 times longer than our own sun!

The largest star is probably about 150-200 times the mass of the Sun. There are only a handful of these hyperstars in our own Milky Way which has over 200 billion stars in it. The Eta Carina nebula appears to have several dozen, mostly unstable stars with masses between 50 and 200 times the sun’s mass. These stars are so masssive that they run through their nuclear fuel in a few million years and explode as hypernovae, many times brighter than ordinary supernovae.

Although hyperstars are rare in a galaxy as large as the Milky Way, brown dwarfs and red dwarfs are not. In fact current searches for exoplanets favor the more numerous red and brown dwarf stars.

Black holes can have any mass from 0.00001 grams to 10 billion times the mass of the Sun…or more. The supermassive black holes are found in the cores of ‘active galaxies’ and quasars. Astronomers have never seen a black hole that is much smaller than the mass of our Sun yet, so we don’t know if they really exist.

Astronomy, Stars

What would happen if two stars collided?

There are several possibilities. If the collision speed is higher than a particular threshold speed, say about 300 miles per second, enough kinetic energy would be imparted to the two masses that the stellar material would dissipate into a vast expanding cloud of gas, never to reassemble itself into a new star.

If the speed were very slow, the stars would merge into a new, more massive, star. The evolution of the new star would begin with a rejuvenated core of fresh fuel since the merging of the two stars would have mixed new hydrogen fuel into the core of the new star.

If the speed of the impact is moderate and off center, the stars will go into a very tight orbit around one another, perhaps even sharing a common gaseous envelope. Over time, the two separate cores would spiral into each other, and you would again be left with one new, massive star. Since the escape velocity of the Sun is about 1.3 million miles per hour, this is about equal to the threshold speed of the impact.

If a smaller star, like a white dwarf or neutron star, smashes into a bigger star, like a red giant, most of the giant’s outer envelope would be blown off as it absorbs the impact. The results get a little more violent when two smaller stars collide. Neutron stars are very small and dense. If a neutron star reaches a certain mass, it will implode and form a black hole. Therefore, if two neutron stars merge but their combined mass is more than the maximum mass a single neutron star can have, they implode into a black hole. If the circumstances are the same when two white dwarf stars collide, they will implode into a neutron star. Here is an artistic rendering of how messy a neutron star-neutron star collision would be. (Credit: ESA)

This artist’s impression shows two tiny but very dense neutron stars at the point at which they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on 17 August 2017 by LIGO–Virgo and Fermi/INTEGRAL respectively. Subsequent detailed observations with many ESO telescopes confirmed that this object, seen in the galaxy NGC 4993 about 130 million light-years from the Earth, is indeed a kilonova. Such objects are the main source of very heavy chemical elements, such as gold and platinum, in the Universe.

A team of astronomers is making a bold prediction: In 2022, give or take a year, a pair of stars will merge and explode, becoming one of the brightest objects in the sky for a short period. It’s notoriously hard to predict when such stellar catastrophes will occur, but this binary pair is engaged in a well-documented dance of death that will inevitably come to a head in the next few years, they say. The researchers began studying the pair, known as KIC 9832227, in 2013 before they were certain whether it was actually a binary or a pulsating star. They found that the speed of the orbit was gradually getting faster and faster, implying the stars are getting closer together. The pair is so close, in fact, they share an atmosphere. KIC 9832227’s behavior reminded the researchers of another binary pair, V1309 Scorpii, which also had a merged atmosphere, was spinning up faster and faster, and exploded unexpectedly in 2008. Now, after 2 years of careful study to confirm the accelerating spin and eliminate alternative explanations, the team predicted in 2017 that the pair will explode as a “red nova”—an explosion caused by a binary merging—in about 5 years’ time.