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A 3-D model of the Milky Way Galaxy using data from Cepheids

Astronomers generate the largest 3D model of Milky Way using Cepheid data

We do not possess a GPS system for our twisted and warped galaxy. As a result, astronomers have to be crafty for pointing our location among stars and producing maps of the Milky Way galaxy. Astronomers from the US and Europe successfully managed to create a 3D model of Milky Way galaxy which is based on the interstellar distance. The study has been published in the Science journal

It draws on the population of stars that are called Cepheids. They are massive, young, pulsing stars having brightness more than that of the Sun. The University of Warsaw ran a sky survey with the help of data from the Optical Gravitational Lensing Experiment from Las Campanas Observatory in Chile. Researchers managed to pick out 2,431 Cepheids through the thick dust and gas of Milky Way and used them for generating the map of the galaxy. 

Dorota Skowron, a researcher with Wroclaw University of Science and Technology and the study’s lead author said that the OGLE project observed the Milky Way’s galactic disk for a period of six years while capturing 206,726 sky images that contained 1,055,030,021 stars. In this they found the Cepheids population to be very useful for the purpose of map plotting since their brightness varies over time. 

This allowed the researchers to observe how bright the star actually is versus how it appears from the Earth. This difference between the two can inform us how far the star is from our Sun. With the help of this fluctuation, scientists produced the galaxy’s 3D model which confirmed the previous work that the galaxy is flared at the edges. They were able to determine the Cepheid’s age where the younger stars were closer to the center and older stars farther away from the galactic disk. 

With the simulation of star formation in the early Milky Way, scientists showed the evolution of the galaxy in past 175 million years with star formation in spiral arms resulting in distribution of Cepheids from 20 million to 260 million years old. Skowron hopes that the paper will be a good initial point for sophisticated modeling of our Galaxy’s past, as the Cepheids are a great testbed for checking the accuracy of the models. 

A study was earlier published in Nature Astronomy which looked at 1339 Cepheids and generated a comprehensive 3D map of Milky Way which found that our galaxy is twisted at the edges. It observed stars from the Wide-field Infrared Survey Explorer (WISE) of NASA. These two studies produced similar results which found about the warped edges of Milky Way. Both the studies relied on the fact that the Cepheids are present on our side of the Milky Way. An important question is whether there is a similar warp in the opposite side too.

Skowron does not think observing the other side will increase the probability of finding Cepheids. The future projects will observe the pulsing star found in our Galaxy called  RR Lyrae. They are present from an earlier time in the Milky Way and can provide another way of mapping the Galaxy. 

Journal Reference: Science journal

binary star system AR Scorpi

Astronomers discover two stars orbiting each other at speeds never observed before

Two dead stars have been found 7800 light-years away from Earth, orbiting each other at incredibly high speeds. Their orbit is so close that astronomers hope to find gravitational waves from the pair in some years when sensitive tools are used. Although we are quite used to observe things on a cosmic level occurring very slowly, the white dwarf binary, ZTF J1539+5027 or J1539, in short, has an orbital period of only 6.91 minutes, shortest for an eclipsing binary. With this close orbit, the binary system could fit inside Saturn. The findings have been published in Nature journal

White dwarfs are dead stars, the progenitor of which had a mass no greater than 10 times of the Sun. If the star is more massive, it converts to a neutron star on its death. On being larger, a black hole is obtained.

When the Sun runs out of hydrogen, it will turn to be a red giant, fusing helium and carbon till their depletion. After this, the outer layers will be blown away, and the remaining shining ultradense core which is the corpse of the dead star is termed as the white dwarf. 

The two stars in J1539 have gone through this process. The primary star has 60 percent mass of the Sun in a core of Earth’s size while the secondary star has only 20 percent mass of the Sun however larger than its companion, hence having a lesser density. This also means that it is less bright so when it moves between its brighter companion and us, it fully obscures the latter from our view so the binary is an eclipsing one. Astronomers found it through the Zwicky Transient Facility of Caltech at Palomar Observatory. 

Kevin Burdge, Caltech physicist explained that when the dimmer star moves in front of the brighter one, it blocks the maximum amount of light creating a seven-minute blinking pattern.

Till now, the gravitational waves have been detected in the final moments of a collision between the massive objects such as black holes or neutron stars. J1539 is relatively lighter and not yet at the condition for stars to merge. However, they are growing closer at the rate of 26 centimetres per day which means that it will take a minimum of 130,000 years for the orbital period to be 5 minutes. In that situation, the mass transfer from the secondary to the primary star will rise. 

There are two possibilities. If a stable mass transfer takes place it will result in the separation of two stars, creating an AM Canum Venaticorum star where the primary star steadily accretes matter from its companion. However, if a stable mass transfer does not occur, the stars will merge creating a R Coronae Borealis variable, a low mass analogue of type la supernova. However, we might not be around to see the final result of the ever decaying orbit of J1539. 

Astronomers believe that the Laser Interferometer Space Antenna (LISA)  once finished will be able to detect the gravitational waves. However, LISA will not launch until 2034 and J1539 will be the strongest signal for it to detect. 

Journal Reference: Nature journal

intergalactic stars

Researchers successfully recreate the sounds of stars through simulation softwares

It is well known that sound cannot propagate in vacuum as it requires a medium for its transmission. Sounds propagate as longitudinal waves in solids and fluids and also as a tranverse wave in solid structures. But scientists have been able to overcome this limitation as they have developed an innovative way for interpreting the signals which have been emitted by cosmos.

Researchers at University of Wisconsin-Madison have separated a different type of resonance which are caused by stars. These vibrations are actually variations in the temperature and the brightness of stars. Very powerful telescopes can spot these vibrations and then recreate the sounds of the stars with the help of computer simulations.

Jacqueline Goldstein, a graduate student in astronomy at University of Wisconsin-Madison said that a cello’s sound is because of its shape and size, similarly the vibrations of the stars are also dependent on their size and composition. Goldstein studies the connection between the structure of stars and their vibrations with the help of the software which simulates many stars and their frequencies. After comparing the simulations to the real stars, she can improve her model and make necessary changes.

For human beings to hear the sounds, the speed of the vibrations have to be increased by thousand to million times, besides repeating the frequencies from minutes to days. These are known as starquakes after their seismic variants on Earth and the field of study is called as astroseismology.

After the fusion of hydrogen in stars to heavier elements in the star cores, plasmic gas vibrates and hence the stars flicker. Researchers can know about the structure of stars through these fluctuations and also the changes which may occur in the star with the passage of time.

Goldstein studies those stars which are bigger than the sun as these are the ones which explode and lead to the formation of black holes, neutron stars and the heavy objects in the cosmos. Scientists want to study about the functioning of these stars and how they make an impact in the expansion and evolution of the universe.

With the help of professors of astronomy, Rich Townsend and Ellen Zweibel, Goldstein has created a computer software named GYRE which is plugged into the simulation software for stars, MESA. These softwares make it possible to develop models of different kinds of stars and observe their vibrations as they may appear to astronomers.

Since, GYRE and MESA are open source programs, they can be accessed freely by the scientists and modified. Goldstein is currently making a modified version of GYRE to take advantage of the data obtained by TESS.