Evening Star Newspaper, January 25, 1931, Page 85

THE SUNDAY STAR, WASHINGTON, D. C, JANUARY 25 193t. 1S ° ~ Taking the Temperarure of Remote Stats How a Little Piece of Wire Measures the Heat of Planets Millions of Miles Away to Find the Surface Temperature of Some Stars Running From Ten Thousand to Forty-One Thosand Degrees Farenheit. . BY G. H. DACY. CIENCE plays doctor, so to say, and, thermometer in hand, takes the tem- perature of remote stars which are so far removed from earth that they are just barely visible to sharp eyes on & dark night. The above is a terse way of describing one of the latest outstanding consummations in astronomical research, with the exception that a thermocouple mounted on the huge 100-inch telescope at the Mount Wilson Observatory of the Carnegie Institution is used as the heat recorder instead of a clinical thermometer. The case is somewhat similar to that of a New York specialist taking the temperature of a patient in San Francisco by reinforced television or some other scientific means—if it were possible. Only that the distance from earth to the star is so much greater than the mileage between the Atlantic and the Pacific, it would merely tire your eyes to read the com- parative figure if we mentioned the same. How is it done, this business of measuring the heat of distant stars and planets? The trick is turned time and again out at the Mount Wilson Observatory by the use of the sensitive thermocouple, which is so effi- cient that it could detect the heat of a candle 100 miles away—were it not for the normal Joss of heat due to absorption by the at- e P Thermocouple, used in taking the tem- perature of distant stars, mounted omn the 100-inch telescope at Mount Wilson Observatory. mosphere which would occur. With this device its originators, Drs. Edison Pettit and 8. B. Nicholoson of the Carnegie In- stitution have for the first time in astronomical annals gauged the radiation of a star of the thirteenth magnitude. Which in its scientific way is as remarkable a record as for “Babe” Ruth to wallop out 200 homers in one and the same seascn. R your information the technical “star- gazers” report that stars of the sixth mag- nitude are just barely visible to the unaided eye. The faintest stars photographed with the 100-inch telescope at Mount Wilson—it is the giant of its species—are of the twenty-first magnitude. Thus a star of the thirteenth magnitude is about one-half way between these extremes, being about 631 times fainter than the faintest star which most of us can discern in the heavens. Which makes the ability of the heat-registering device which responds to the heat from such a star the more astonish- " ing to laity and the scientific world alike. This exploit becomes even more impressive when you realize that a star of the sixth magnitude, which is just visible from the earth, + yradiates upon the United States only about as much heat as the sun radiates upon one square yard of the earth’s surface. The heat record from such a star as detected by the thermo- couple amounts to the bewildering figure of one-half of one-millionth of a degree Fahren- heit. The electric current which it generates in the thermocouple circuit as recorded by the supplementary galvanometer is approximately one-twenty-billionth of an ampere. The sensi- tivity of this recording thermocouple is in fact 80 great that where bright stars near the hori- zon are being studied it will detect the minute change in brightness which occurs in 60 seconds, As described by its inventors, this wonderful heat-recording apparatus consists of two minute wires of bismuth and a bismuth alloy contain- ing one-twentieth tin fused together at the ends. These wires are linked in electrical circuit with a galvanometer so that the currents de- veloped when the junctions are heated sep- arately flow in opposite directions. Small metal disks fused over the junctions of the thermo- couple wires are coated with a mixture of lamp-black and platinum-black. These black plates absorb radiation from celestial objects very well and convert it into heat. The ther- mocouple is operated within a vacuum in order to control the conduction losses of star heat. The weight of one of these complete thermo- couples, including its metal receiver and the connecting wires, is less than one one-thou- sandth of the total weight of a drop of water. When the heat recorder is yoked for service, it is mounted upon the 100-inch telescope which is trained uporfthe star to be studied so that its rays fall upon the concave mirror of the telescope and are focused upon one of the junctions of the themocouple. Electric cur- rents produced in this manner are propor- tional to the amount of heat received by the thermocouple, so that the galvanometer deflec- tions are an accurate measure of the heat re- ceived from the star under examination. These galvanometer deflections are recorded photo- graphically and can be measured with extreme accuracy under the most favorable conditions. 'HIS ingenious star heat sleuth has disclosed new facts about old stars. For example, it has divuiged that the surface temperatures of the stars range from 41,000 degrees Fahrenheit for the very blue stars, through 10,000 degrees Fahrenheit for those of the class to which the sun belongs, on down to 2,800 degrees Fahren- heit for the bright red, long period variable stars. Remarkable although it be, the hottest stars do not necessarily radiate the greatest heat. Although they may radiate the maxi- mum heat per unit of area, a cooler star may be so much larger that its total radiation ex- ceeds that of the very hot star. Some stars resemble electric lights, being small, hot and bright, while others are like electric stoves, being large, cool, faint as seen from the earth, yet capable of radiating enormous amounts of heat. Chi Cygni is the coolest star which the Car- negie experimenters have measured. At mini- mum brightness its temperature is equal to that of a hot star 50,000 times as bright. There may be cooler stars which give off practically no light, yet which radiate much heat, but thus far astronomical ferrets have failed to find them. Science can calculate the diameter of a re- mote star very accurately if its temperature and total radiated energy are known. If the distance to the star is known, its total radia- tion can be determined by the thermocouple. Even if the distance to the star is unknown, its angular diameter can be obtained. By use of the stellar interferometer, the angular diam- eters of some of the largest stars have been measured directly. The star Sirius, although hot and close to earth, is so small that its total heat radiation approximates that regis- This record telescope, with the supplementary thermocouple, was used in taking the temperature of a star whose surfaces were as- hot at 41,000 degrees F. tered from the big star Betelgeuse, which' is over 30 times more remote. The diameter of Sirius is one and one-half times that of the sun, while the diameter of Betelgeuse is 200 times greater than that of the sun. In addition to taking the temperature of both the near and far stars, the Carnegie In- stitution scientists have also measured the heat of the planets and various regions of the moon. The moon and the other planets are much cooler than the stars. The light which we see when we gaze at them is nothing more than reflected sunshine. They radiate energy waves, known as planetary heat, which are too long in wave length to be visible to the human eye. However, by the use of thin glass screens set up in the path of radiation it is simple for science to separate planetary heat from re- flected sunlight. The former is absorbed, while the latter is transmitted through the glass. Then as the distance of the planet under ob- servation and the area of the surface which is sending heat to the registering thermocouple are known, the temperature of that planet can be calculated from the amount of radiation An estimated chart of the temperatures at various parts of the moon, based on the Carnegie Institute astronomical research. " transmitted to the earth per unit area. Th@ most uncertain part of this calculation occurs in the correction which must be made for losse§ when the rays are passing through the earth’s atmosphere. Atmospheric conditions on thé " planets differ so measurably from those og earth that the temperatures obtained by place ing a thermometer near their surfaces would . probably differ greatly from the results obtained by the radiation methods described in this - article. The maximum temperature recorded on Mers cury is about 800 degrees Fahrenheit, the dise tribution of radiation over its surface being quite similar to that of the moon. The teme - peratures on Mars are somewhat like those om er:th, but vary greatly with the season and the hour of the day. The outer planets, unles§ they give off heat from their interiors, are very cold. pecause of their remoteness from the sumy Jupi:er has long been supposed to be warmgy but the recent thermocouple observations show that its temperature is approximately minu§ 216 degrees Fahrenheit. The nocturnal temie perature in the atmosphere of cloud-covered Venus is about minus 9 degrees Fahrenheit. a DRS PETTIT AND NICHOLSON repord; after seven years of stellar observations, " that the temperature at the point on the moon ' tion where the sun is directly overhead is aboud 244 degrees Fahrenheit at the time of full moon and 149 degrees Fahrenheit during the first and third quarters. This variation ap= parently is caused by the irregularities in thé moon’s surface, which tend to increase radiae gent out in the direction of the luna# zenith and decreases that emitted toward theé horizon. The true temperature of the moon He§ somewhere between the figures mentioned above. - e Science appreciates that much more is to bé learned about both the planets and the moo® frcm the invaluable Carnegie Institution ree search. The cooling and heating of the moon is governed by the amounts of sunlight which are flooded upon it. The investigators hope to - discover eventually what kind of rocks occus in the surface of the distant moon. In fact, laboratory tests have already been made with granite, lava, quartz sand and various other materials by heating and cooling them, observe ing the rates at which their temperatures change, in order to compare these results with those obtained from scientific observations with the thermocouple of the moon itself. The variation in the radiation and temperae ture over the sun-lit side of the full moon was - obtained by allowing its image to drift acrospe¥ the receiver of the thermocouple while a moving ghotographic plate traced the deflece tions of the galvanometer which resulted. The planetary heat varies from a practically un- measurable quantity at the edge to a maximum at the center. Observations of a lunar eclipse several years ago showed that the temperature at the observed point fell a total of 335 degrees Fahrenheit during the eclipse, the greater part of this drop occurring during the partial phase of that particular eclipse. This record is in marked contrast to the observations of solaw. eclipses on the earth, which featured falls in temperature of only 4 to 5 degrees Fahrenheit. The mountains in the moon—yes, there is & lot yet to be:.learned about them, but that is just ancther story.