The daytime surface temperature is about 80 F during rare summer days, to -200 F at the poles in winter. The air temperature, however, rarely gets much above 32 F.
The temperatures on the two Viking landers, measured at 1.5 meters above the sursurface, range from + 1° F, ( -17.2° C) to -178° F (-107° C). However, the temperature of the surface at the winter polar caps drop to -225° F, (-143° C) while the warmest soil occasionally reaches +81° F (27° C) as estimated from Viking Orbiter Infrared Thermal Mapper.
In 2004, the Spirit rover recorded the warmest temperature around +5 C and the coldest is -15 Celsius in the Guisev Crater.
The Curiosity Rover landed in August 2012 and has been steadily building up an enormous data base of meteorological conditions and measurements. Here from its equatorial location in Gale Crater are its temperature readings. Credit: NASA/JPL-Caltech/CAB(CSIC-INTA)/FMI/Ashima Research. This pair of graphs shows about one-fourth of a Martian year’s record of temperatures (in degrees Celsius) measured by the Rover Environmental Monitoring Station (REMS) on NASA’s Curiosity rover. The data are graphed by sol number (Martian day, starting with Curiosity’s landing day as Sol 0), for a period from mid-August 2012 to late February 2013, corresponding to late winter through the end of spring in Mars’ southern hemisphere.
This colorful view of Mercury was produced by using images from the color base map imaging campaign during MESSENGER’s primary mission. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up the planet’s surface. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Carnegie Institution of Washington.
Most likely Mercury and Venus have no moons because they are too close to the Sun.
Any moon with too great a distance from these planets would be in an unstable orbit and it would be captured by the Sun. If they were too close to these planets they would be destroyed by tidal gravitational forces. The zones where moons around these planets could be stable over billions of years is probably so narrow that no body was ever captured into orbit, or created in situ, when the planets were first being accreted.
What are the specifics? Lets do the calculation of forces!
Mercury: How far from Mercury would a satellite be at which point the gravity force of the sun equals that of mercury?
G(Mp)m/rs^2 = G(Ms)m/R^2
were M is the masss of the sun, m is the mass of the satellite, rs is the distance of the satellite from the center of mercury and R is the distance from Mercury to the sun. The value for the satellite mass, m, cancils, as does the constant of gravity, G, so for R = 35 million km, Ms = 2×10^30 kg, Mp = 3.3×10^23 kg, we get with some algebra that rs = 14,000 km.
The tidal limit for the satellite, which is the point at which the tidal gravity force of Mercury shatters the satellite, is given by the formula R = 1.26 x Rm (Rho m/Rho M)^1/3 where Rho m is the satellites density and Rho M is the planets density. For a satellite made of mercury material the densities are the same so the tidal radius is 1.26 x Mercury’s radius of 2440 km, or 3070 kilometers.
So the roughly stable orbit range for such a satellite is between 3000 km and 14,000 km from the center of Mercury, or between 600 and 11,000 km from its surface. For Venus, the same calculation woud give a significantly wider orbit range from 7600 km to 162,000 km from the planet’s center, or 1600 to 156,000 km from its surface.
Clearly, Venus is a far more promissing place to stuff one or more moons, so it is a puzzle why it has none, unless the catastrophic impact that tilted Venus’s rotation axis also stripped off any primordial, orbiting moons.
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.
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.
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.
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.
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:
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.
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.
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.
Illustration of matter in an accretion disk falling into a black hole. (Credit: NASA’s Goddard Space Flight Center/Jeremy Schnittman). The actual image of the disk will be distorted due to the intense gravitational field and will probably look like the following image.
Outside the black hole, it depends on what form the matter takes. If it happens to be in the form of gas that has been orbiting the black hole in a so-called accretion disk, the matter gets heated to very high temperatures as the individual atoms collide with higher and higher speed producing friction and heat. The closer the gas is to the black hole and its Event Horizon, the more of the gravitational energy of the gas gets converted to kinetic energy and heat. Eventually the atoms collide so violently that they get stripped of their electrons and you then have a plasma. All along, the gas emits light at higher and higher energies, first as optical radiation, then ultraviolet, then X-rays and finally, just before it passes across the Event Horizon, gamma rays.
Here is what a model of such a disk looks like based on a typical calculation, in this case by physicist Kovak Zoltan (Phys Rev D84, 2011, pp 24018) for a 2 million solar mass black hole accreting mass at a rate of 2.5 solar masses every million years. Even around massive black holes, temperatures run very hot. The event horizon for this black hole is at a distance of 6 million kilometers. The first mark on the horizontal axis is ‘5’ meaning 5 times the horizon radius or a distance of 30 million km from the center of the black hole. This is about the distance from our sun to mercury!
If the matter is inside a star that has been gravitationally captured by the black hole, the orbit of the star may decrease due to the emission of gravitational radiation over the course of billions of years. Eventually, the star will pass so close to the black hole that its fate is decided by the mass of the black hole. If it is a stellar-mass black hole, the tidal gravitational forces of the black hole will deform the star from a spherical ball, into a football-shaped object, and then eventually the difference in the gravitational force between the side nearest the black hole, and the back side of the star, will be so large that the star can no longer hold itself together. It will be gravitationally shredded by the black hole, with the bulk of the star’s mass going into an accretion disk around the black hole. If the black hole has a mass of more than a billion times that of the sun, the tidal gravitational forces of the black hole are weak enough that the star may pass across the Event Horizon without being shredded. The star is, essentially, eaten whole and the matter in the star does not produce a dramatic increase in radiation before it enters the black hole. Here is an artist version of such a tidal encounter.
Once inside a black hole, beyond the Event Horizon, we can only speculate what the fate of captured matter is. General relativity tells us that there are two kinds of black holes; the kind that do not rotate, and the kind that do. Each of these kinds has a different anatomy inside the Event Horizon.For the non-rotating ‘Schwarzschild black hole’, there is no way for matter to avoid colliding with the Singularity. In terms of the time registered by a clock moving with this matter, it reaches the Singularity within a few micro seconds for a solar-massed black hole, and a few hours for a supermassive black hole. We can’t predict what happens at the Singularity because the theory says we reach a condition of infinite gravitational force.
For the rotating ‘ Kerr Black holes’, the internal structure is more complex, and for some ingoing trajectories for matter, you could in principle avoid colliding with the Singularity and possibly reemerge from the black hole somewhere else, or at some very different future time thousands or billions of years after you entered.
Some exotic theories say that you reemerge in another universe entirely, but physicists now don’t believe that interpretation is accurate. The problem is that for black holes created by real physical events, the interior of a black hole is awash with gravitational radiation which makes the geometry of space-time very unstable, preventing just these kinds of trips.
For the simplist non-rotating Schwarschild black holes, even they offer a mind-numbing prospect. The mathematics says that outside the event horizon, a particle will experience space and time normally. The particle (and you!) can travel freely in space along the R, radial coordinate, but have no control over your progression in time along the T coordinate. You can speed it up or slow it down a bit through the time dilation effect of high-sped travel, but you can not travel backwards in time. At the event horizon, something amazing happens. The mathematical variables we have been using for time and space, that is R and T, reverse their rolls in the equations that define the separation between points in spacetime. What this means is that the space coordinate, R, behaves like a time coordinate so that you have no freedom to maneuver and not be crushed at the Singularity at R=0. Meanwhile you have some freedom to move along the T coordinate as though it acted like the old familiar space coordinate out side the event horizon.
For Schwarschild black holes that form from supernovae, you have another problem. The event horizon in the mathematics only appears a LONG time after the implosion of matter. In fact it is what mathematicians call an asymptotic feature of the collapsing spacetime. What this means is that if you fell into the black hole long after the supernova created it, the collapse is still going on in the frame of someone far away with the surface of the star trying to pass inside the horizon, but this process has not yet completed. For you falling in, the bulk of the star is still outside the horizon and the black hole has not yet formed! The time dilation effect is so extreme at the horizon that the star literally freezes its motion from the standpoint of the distant observer and becomes a frozen-in-time, black star. As seen from the outside, it will take an eternity for you to actually reach the horizon, but from your frame of reference, it will only take an hour or less depending on where you start! Once you pass inside the horizon, the time to arriving at the singularity is approximately the gravitational free-fall time from the horizon distance. For a supermassive black hole this could take hours, but for a solar mass black hole this takes about 10 microseconds!
An astronomer's point-of-view on matters of space, space travel, general science and consciousness
