Colors Of Stars Research Articles

Data-driven Stellar Intrinsic Colors and Dust Reddenings for Spectrophotometric Data: From the Blue-edge Method to a Machine Learning Approach

Intrinsic colors (ICs) of stars are essential for studies on both stellar physics and dust reddening. In this work, we developed an XGBoost model to predict ICs with the atmospheric parameters T eff, logg , and [M/H]. The model was trained and tested for three colors at Gaia and Two Micron All Sky Survey bands with 1,040,446 low-reddening sources. The atmospheric parameters were determined by the Gaia DR3 General Stellar Parameterizer from Photometry (GSP-phot) module and were validated by comparing with the Apache Point Observatory Galactic Evolution Experiment and Large Sky Area Multi-Object fiber Spectroscopic Telescope. We further confirmed that the biases in GSP-phot parameters, especially for [M/H], do not present a significant impact on the IC prediction. The generalization error of the model estimated by the test set is 0.014 mag for (GBP−GRP)0 , 0.050 mag for (GBP−KS)0 , and 0.040 mag for (J−KS)0 . The model was applied to a sample containing 5,714,528 reddened stars with stellar parameters from R. Andrae et al. to calculate ICs and reddenings. The high consistency in the comparison of E(J − K S) between our results and literature values further validates the accuracy of the XGBoost model. The variation of E(G BP − K S)/E(G BP − G RP), a representation of the extinction law, with Galactic longitude is found on large scales. This work preliminarily presents the feasibility and accuracy of the machine learning approach for IC and dust reddening calculation, whose products could be widely applied to spectrophotometric data. The data sets and trained model can be accessed via Zenodo, doi:10.5281/zenodo.12787594. Models for more bands will be completed in following work.

The JWST Resolved Stellar Populations Early Release Science Program. VI. Identifying Evolved Stars in Nearby Galaxies

We present an investigation of evolved stars in the nearby star-forming galaxy Wolf–Lundmark–Melotte (WLM), using Near-Infrared Camera (NIRCam) imaging from the JWST Resolved Stellar Populations Early Release Science program. We find that various combinations of the F090W, F150W, F250M, and F430M filters can effectively isolate red supergiants and thermally pulsing asymptotic giant branch (TP-AGB) stars from one another, while also providing a reasonable separation of the primary TP-AGB subtypes: carbon-rich C-type stars and oxygen-rich M-type stars. The classification scheme we present here agrees very well with the well-established Hubble Space Telescope (HST) medium-band filter technique. The ratio of C to M-type stars is 0.8 ± 0.1 for both the new JWST and the HST classifications, which is within 1σ of empirical predictions from optical narrowband CN and TiO filters. The evolved star colors show good agreement with the predictions from the PARSEC + COLIBRI stellar evolutionary models, and the models indicate a strong metallicity dependence that makes stellar identification even more effective at higher metallicity. However, the models also indicate that evolved star identification with NIRCam may be more difficult at lower metallicities. We test every combination of NIRCam filters using the models and present additional filters that are also useful for evolved star studies. We also find that ≈90% of the dusty evolved stars are carbon rich, suggesting that carbonaceous dust dominates the present-day dust production in WLM, similar to the findings in the Magellanic Clouds. These results demonstrate the usefulness of NIRCam in identifying and classifying dust-producing stars without the need for mid-infrared data.

Linking Transients to their Host Galaxies: II. A Comparison of Host Galaxy Properties and Rate Dependencies across Supernova Types

Abstract We use the latest dataset of supernova (SN) host galaxies to investigate how the host properties – stellar mass, star formation rate, metallicity, absolute magnitude, and colour – differ across SN types, with redshift-driven selection effects controlled. SN Ib and Ic host galaxies, on average, are more massive, metal-rich, and redder than SN II hosts. For subtypes, SN Ibn and Ic-BL have bluer hosts than their normal SN Ib and Ic siblings; SN IIb has consistent host properties with SN Ib, while hosts of SN IIn are more metal-rich than those of SN II. Hydrogen-deficient superluminous supernovae feature bluer and lower luminosity hosts than most subtypes of core-collapse supernova (CC SN). Assuming simple proportionality of CC SN rates and host star formation rates (SFRs) does not recover the observed mean host properties; either a population of long-lived progenitors or a metallicity-dependent SN production efficiency better reproduces the observed host properties. Assuming the latter case, the rates of SN II are insensitive to host metallicity, but the rates of SN Ib and Ic are substantially enhanced in metal-rich hosts by a factor of ∼10 per dex increase in metallicity. Hosts of SN Ia are diverse in their observed properties; subtypes including SN Ia-91T, Ia-02cx, and Ia-CSM prefer star-forming hosts, while subtypes like SN Ia-91bg and Ca-rich prefer quiescent hosts. The rates of SN Ia-91T, Ia-02cx, and Ia-CSM are closely dependent on, or even proportional to, their host SFRs, indicating relatively short-lived progenitors. Conversely, the rates of SN Ia-91bg and Ca-rich transients are proportional to the total stellar mass, favoring long-lived progenitors.

EPOCHS Paper V. The dependence of galaxy formation on galaxy structure at z < 7 from JWST observations

ABSTRACT We measure the broad impact of galaxy structure on galaxy formation by examining the ongoing star formation and integrated star formation history as revealed through the stellar masses of galaxies at z < 7 based on JWST CEERS data from the Extended Groth Strip (EGS). Using the morphological catalog of 3965 visually classified JWST galaxies from Ferreira et al. (2023), we investigate the evolution of stars, and when they form, as a function of morphological type as well as galaxies classified as passive and starburst through spectral energy distributions. Although disc galaxies dominate the structures of galaxies at z < 7, we find that these discs are in general either ‘passive’, or on the main sequence of star formation, and do not contain a large population of starburst galaxies. We also find no significant correlation between morphological type and the star formation rate or colours of galaxies at z < 7. In fact, we find that the morphologically classified ‘spheroids’ tend to be blue and are not found to be predominately passive systems at z > 1.5. We also find that the stellar mass function for disc galaxies does not evolve significantly during this time, whereas other galaxy types, such as the peculiar population, evolve dramatically, declining at lower redshifts. This indicates that massive peculiars are more common at higher redshifts. We further find that up to z ∼ 7, the specific star formation rate (sSFR) does not vary with visual morphology, but strongly depends on stellar mass and internal galaxy mass density. This demonstrates that at early epochs galaxy assembly is a mass-driven, rather than a morphologically driven process. Quenching of star formation is therefore a mass-dominated process throughout the universe’s history, likely due to the presence of supermassive black holes.

Colour and temperature of the stars: a demonstration using Arduino

Teaching the colour of stars is not as trivial as one might think. It can be challenging for students to grasp that the colour of stars follows a temperature sequence. This paper introduces a simple experimental setup for instructing the correlation between a star’s colour and its temperature. Furthermore, the experimental setup facilitates the exploration of the topic of colour addition, demonstrating to students how to replicate the colour of a star—a spectrum colour—by employing an RGB LED that emits only primary colours (red, green, and blue). The experiment utilised an Arduino microcontroller board in conjunction with RGB LEDs and an LCD display. The activity was conducted with 53 7th-grade students from a private school in Portugal results suggest a positive reception, indicating success in both motivational and cognitive aspects. The overall outcomes underscore the effectiveness of the activity in imparting new knowledge to students.

A Measurement of the Assembly of Milky Way Analogs at Redshifts 0.5 < z < 2 with Resolved Stellar Mass and Star Formation Rate Profiles

The resolved mass assembly of Milky Way–mass galaxies has been previously studied in simulations, the local Universe, and at higher redshifts using infrared (IR) light profiles. To better characterize the mass assembly of Milky Way analogs (MWAs), as well as their changes in star formation rate (SFR) and color gradients, we construct resolved stellar mass and SFR maps of MWA progenitors selected with abundance matching techniques up to z ∼2 using deep, multiwavelength imaging data from the Hubble Frontier Fields. Our results using stellar mass profiles agree well with previous studies that utilize IR light profiles, showing that the inner 2 kpc of the galaxies and the regions beyond 2 kpc exhibit similar rates of stellar mass growth. This indicates the progenitors of MWAs from z ∼ 2 to the present do not preferentially grow their bulges or their disks. The evolution of the SFR profiles indicates a greater decrease in SFR density in the inner regions versus the outer regions. Sérsic parameters indicate modest growth in the central regions at lower redshifts, perhaps indicating slight bulge growth. However, the Sérsic index does not rise above n ∼ 2 until z < 0.5, meaning these galaxies are still disk-dominated systems. We find that the half-mass radii of the MWA progenitors increase between 1.5 < z < 2, but remain constant at later epochs (z < 1.5). This implies mild bulge growth since z ∼ 2 in MWA progenitors, in line with previous MWA mass assembly studies.

Revealing ringed galaxies in group environments

Aims. We explore galaxies with ringed structures inhabiting poor and rich groups with the aim of assessing the effects of local density environments on ringed galaxy properties. Methods. We identified galaxies with inner, outer, nuclear, inner+outer (inner and outer rings combined), and partial rings that reside in groups by cross-correlating a sample of ringed galaxies with a group catalog obtained from Sloan Digital Sky Survey (SDSS). The resulting sample was divided based on group richness, with groups having 3 ≤ Nrich ≤ 10 members classified as poor, while groups having 11 ≤ Nrich ≤ 50 were classified as rich. To quantify the effects of rings and the role of local density environment on galaxy properties, we constructed a suitable control sample for each catalog of ringed galaxies in poor and rich groups, consisting of non-ringed galaxies with similar values for the redshift, magnitude, morphology, group masses, and environmental density distributions as those of ringed ones. We explored the occurrence of ringed galaxies in poor and rich groups and analyzed several galaxy properties, such as star formation activity, stellar populations, and colors, with respect to the corresponding comparison samples. Results. We obtained a sample of 637 ringed galaxies residing in groups. We found that about 76% of these galaxies inhabit poor groups, whereas only about 24% are present in rich groups. Inner rings are prevalent in both rich and poor groups, while nuclear rings are the least common in both groups. Regarding the control sample, about 81% galaxies are found in poor groups and about 19% in rich ones. We find that the percentages of ringed galaxies with bar structures are similar, regardless of whether the group is rich or poor. In addition, ringed galaxies inhabiting groups display a reduction in their star formation activity and aged stellar populations, compared to non-ringed ones in the corresponding control samples. However, the star formation rate is higher for nuclear rings in poor groups than for other types. This disparity may stem from the environmental influence on the internal processes of galaxies, either enhancing or diminishing star formation. Ringed galaxies also show an excess of red colors and tend to populate the green valley and the red sequence of color-magnitude and color-color diagrams, with a surplus of galaxies in the red sequence, while non-ringed galaxies are found in the green valley and the blue region. These trends are more significant in galaxies with ringed structures residing in rich groups. Our findings provide valuable insights into the relationship between ringed structures and their surrounding environments, paving the way for further explorations in this area of study.

Stellar Loci. VII. Photometric Metallicities of 5 Million FGK Stars Based on GALEX GR6+7 AIS and Gaia EDR3

We combine photometric data from GALEX GR6+7 All-Sky Imaging Survey and Gaia Early Data Release 3 with stellar parameters from the SAGA and PASTEL catalogs to construct high-quality training samples for dwarfs (0.4 < BP − RP < 1.6) and giants (0.6 < BP − RP < 1.6). We apply careful reddening corrections using empirical temperature- and extinction-dependent extinction coefficients. Using the two samples, we establish a relationship between stellar loci (near-ultraviolet (NUV)−BP versus BP − RP colors), metallicity, and M G . For a given BP − RP color, a 1 dex change in [Fe/H] corresponds to an approximately 1 magnitude change in NUV − BP color for solar-type stars. These relationships are employed to estimate metallicities based on NUV − BP, BP − RP, and M G . Thanks to the strong metallicity dependence in the GALEX NUV band, our models enable a typical photometric-metallicity precision of approximately σ [Fe/H] = 0.11 dex for dwarfs and σ [Fe/H] = 0.17 dex for giants, with an effective metallicity range extending down to [Fe/H] = −3.0 for dwarfs and [Fe/H] = −4.0 for giants. We also find that the NUV-band-based photometric-metallicity estimate is not as strongly affected by carbon enhancement as previous photometric techniques. With the GALEX and Gaia data, we have estimated metallicities for about 5 million stars across almost the entire sky, including approximately 4.5 million dwarfs and 0.5 million giants. This work demonstrates the potential of the NUV band for estimating photometric metallicities, and sets the groundwork for utilizing the NUV data from space telescopes such as the upcoming Chinese Space Station Telescope.

Collisional Shaping of Nuclear Star Cluster Density Profiles

A supermassive black hole surrounded by a dense, nuclear star cluster resides at the center of many galaxies. In this dense environment, high-velocity collisions frequently occur between stars. About 10% of the stars within the Milky Way’s nuclear star cluster collide with other stars before evolving off the main sequence. Collisions preferentially affect tightly bound stars, which orbit most quickly and pass through regions of the highest stellar density. Over time, collisions therefore shape the bulk properties of the nuclear star cluster. We examine the effect of collisions on the cluster’s stellar density profile. We show that collisions produce a turning point in the density profile, which can be determined analytically. Varying the initial density profile and collision model, we characterize the evolution of the stellar density profile over 10 Gyr. We find that old, initially cuspy populations exhibit a break around 0.1 pc in their density profile, while shallow density profiles retain their initial shape outside of 0.01 pc. The initial density profile is always preserved outside of a few tenths of a parsec irrespective of initial conditions. We generalize this model to an arbitrary galactic nucleus and show that the location of the collisional break can be simply estimated from the nuclear properties. Lastly, we comment on the implications of collisions for the luminosity and color of stars in the collisionally shaped inner cluster.

The 3D geometry of reflection nebulae IC 59 and IC 63 with their illuminating star gamma Cas

ABSTRACT The early-type star gamma Cas illuminates the reflection nebulae IC 59 and IC 63, creating two photodissociation regions (PDRs). Uncertainties about the distances to the nebulae and the resulting uncertainty about the density of the radiation fields incident on their surfaces have hampered the study of these PDRs during the past three decades. We employed far-ultraviolet (UV) – optical nebula – star colour differences of dust-scattered light to infer the locations of the nebulae relative to the plane of the sky containing gamma Cas, finding IC 63 to be positioned behind the star and IC 59 in front of the star. To obtain the linear distances of the nebulae relative to gamma Cas, we fit far-infrared archival Herschel flux data for IC 59 and IC 63 with modified blackbody curves and relate the resulting dust temperatures with the luminosity of gamma Cas, yielding approximate distances of 4.15 pc for IC 59 and 2.3 pc for IC 63. With these distances, using updated far-UV flux data in the 6–13.6 eV range for gamma Cas with two recent determinations of the interstellar extinction for gamma Cas, we estimate that the far-UV radiation density at the surface of IC 63 takes on values of G0 = 58 or G0 = 38 with respective values for E(B − V) for gamma Cas of 0.08 and 0.04 mag. This is a substantial reduction from the range 150 ≤ G0 ≤ 650 used for IC 63 during the past three decades. The corresponding, even lower new values for IC 59 are G0 = 18 and G0 = 12.

Zero-age horizontal branch models for −2.5 ≤ [Fe/H] ≤−0.5 and their implications for the apparent distance moduli of globular clusters

ABSTRACT Grids of zero-age horizontal branch (ZAHB) models are presented, along with a suitable interpolation code, for −2.5 ≤ [Fe/H] ≤ −0.5, in steps of 0.2 dex, assuming Y = 0.25 and 0.29, [O/Fe] = +0.4 and +0.6, and [m/Fe] = +0.4 for all of the other α-elements. The HB populations of 37 globular clusters (GCs) are fitted to these ZAHBs to derive their apparent distance moduli, (m − M)V. With few exceptions, the best estimates of their reddenings from dust maps are adopted. The distance moduli are constrained using the prediction that (MF606W − MF814W)0 colours of metal-poor, main-sequence stars at $M_{F606W} \mathrel {\rm{{\gt }\lower.5 ex\rm{\sim }}}5.0$ have very little sensitivity to [Fe/H]. Intrinsic (MF336W − MF606W)0 colours of blue HB stars, which provide valuable connections between GCs with exclusively blue HBs and other clusters of similar metallicity that also have red HB components, limit the uncertainties of relative (m − M)V values to within ±0.03–0.04 mag. The ZAHB-based distances agree quite well with the distances derived by Baumgardt &amp; Vasiliev. Their implications for GC ages are briefly discussed. Stellar rotation and mass loss appear to be more important than helium abundance variations in explaining the colour–magnitude diagrams of second-parameter GCs (those with anomalously very blue HBs for their metallicities).

Astrophysical Distance Scale. VII. A Self-consistent, Multiwavelength Calibration of the Slopes and Relative Zero Points for the Run of Luminosity with the Color of Stars Defining the Tip of the Red Giant Branch

Given the recent successful launch of the James Webb Space Telescope, determining robust calibrations of the slopes and absolute magnitudes of the near- to mid-infrared tip of the red-giant branch (TRGB) will be essential to measuring precise extragalactic distances via this method. Using ground-based data of the Large Magellanic Cloud from the Magellanic Clouds Photometric Survey along with near-infrared (NIR) data from 2MASS and mid-infrared (MIR) data collected as a part of the SAGE survey using the Spitzer Space Telescope, we present slopes and zero-points for the TRGB in the optical (VI), NIR (JHK), and MIR ([3.6] and [4.5]) bandpasses. These calibrations utilize stars +0.3 ± 0.1 mag below the tip, providing a substantial statistical improvement over previous calibrations which only used the sample of stars narrowly encompassing the tip.

A deep dive into the Type II Globular Cluster NGC 1851

Abstract About one-fifth of the Galactic globular clusters (GCs), dubbed Type II GCs, host distinct stellar populations with different heavy elements abundances. NGC 1851 is one of the most studied Type II GCs, surrounded by several controversies regarding the spatial distribution of its populations and the presence of star-to-star [Fe/H], C+N+O, and age differences. This paper provides a detailed characterization of its stellar populations through Hubble Space Telescope (HST), ground-based, and Gaia photometry. We identified two distinct populations with different abundances of s-process elements along the red-giant branch (RGB) and the sub-giant branch (SGB) and detected two sub-populations among both s-poor (canonical) and s-rich (anomalous) stars. To constrain the chemical composition of these stellar populations, we compared observed and simulated colors of stars with different abundances of He, C, N, and O. It results that the anomalous population has a higher CNO overall abundance compared to the canonical population and that both host stars with different light-element abundances. No significant differences in radial segregation between canonical and anomalous stars are detected, while we find that among their sub-populations, the two most chemical extremes are more centrally concentrated. Anomalous and canonical stars show different 2D spatial distributions outside ∼3 arcmin, with the latter developing an elliptical shape and a stellar overdensity in the northeast direction. We confirm the presence of a stellar halo up to ∼80 arcmin with Gaia photometry, tagging 14 and five of its stars as canonical and anomalous, respectively, finding a lack of the latter in the south/southeast field.

Do Minor Interactions Trigger Star Formation in Galaxy Pairs?

We analyze the galaxy pairs in a set of volume limited samples from the Sloan Digital Sky Survey to study the effects of minor interactions on the star formation rate (SFR) and color of galaxies. We carefully design control samples of isolated galaxies by matching the stellar mass and redshift of the minor pairs. The SFR distributions and color distributions in the minor pairs differ from their controls at >99% significance level. We also simultaneously match the control galaxies in stellar mass, redshift and local density to assess the role of the environment. The null hypothesis can be rejected at >99% confidence level even after matching the environment. Our analysis shows a quenching in the minor pairs where the degree of quenching decreases with the increasing pair separation and plateaus beyond 50 kpc. We also prepare a sample of minor pairs with H α line information. We calculate the SFR of these galaxies using the H α line and repeat our analysis. We observe a quenching in the H α sample too. We find that the majority of the minor pairs are quiescent systems that could be quenched due to minor interactions. Combining data from the Galaxy Zoo and Galaxy Zoo 2, we find that only ∼1% galaxies have a dominant bulge, 4%–7% galaxies host a bar and 5%–10% of galaxies show active galactic nucleus (AGN) activity in minor pairs. This indicates that the presence of bulge, bar or AGN activity plays an insignificant role in quenching the galaxies in minor pairs. The more massive companion satisfies the criteria for mass quenching in most of the minor pairs. We propose that the stripping and starvation likely caused the quenching in the less massive companion at a later stage of evolution.

Gaia17bpp: A Giant Star with the Deepest and Longest Known Dimming Event

We report the serendipitous discovery of Gaia17bpp/2MASS J19372316+1759029, a binary star with a deep single large-amplitude dimming event of ∼4.5 mag that lasted over 6.5 yr. Using the optical-to-IR spectral energy distribution (SED), we constrain the primary star to be a cool giant M0III star with effective temperature T eff = 3850 K and radius R = 58 R ⊙. Based on the SED fitting, we obtained a bimodal posterior distribution of primary stellar masses with a stronger preference for a 1.5 M ⊙ mass star. Within the last 66 yr of photometric coverage, no other significant dimming events of this depth and duration were identified in the optical light curves. Using a Gaussian process, we fit a generalized Gaussian distribution to the optical and IR light curves and conclude that the dimming event exhibits moderate asymmetries from optical to IR. At the minimum of the dimming event, the Wide-field Infrared Survey Explorer color (W1–W2) differed by ∼0.2 mag relative to the primary star color outside the dimming event. The ingress and egress colors show a shallow reddening profile. We suggest that the main culprit of the dimming event is likely due to the presence of a large, optically thick disk transiting the primary giant star. By fitting a monochromatic transit model of an oblate disk transiting a star, we found good agreement with a slow-moving (0.005 km s−1) disk with a ∼1.4 au radius. We propose that Gaia17bpp belongs to a rare binary star population similar to the ϵ Aurigae system, which consists of a secondary star enshrouded by an optically thick debris disk.

Multiple Stellar Populations in Metal-poor Globular Clusters with JWST: A NIRCam View of M92

Recent work on metal-intermediate globular clusters (GCs) with [Fe/H] = −1.5 and −0.75 has illustrated the theoretical behavior of multiple populations in photometric diagrams obtained with the JWST. These results are confirmed by observations of multiple populations among the M dwarfs of 47 Tucanae. Here we explore multiple populations in metal-poor GCs with [Fe/H] = −2.3. We take advantage of synthetic spectra and isochrones that account for the chemical composition of multiple populations to identify photometric diagrams that separate the distinct stellar populations of GCs. We derive high-precision photometry and proper motion for main-sequence (MS) stars in the metal-poor GC M92 from JWST and Hubble Space Telescope images. We identify a first-generation (1G) and two main groups of second-generation (2GA and 2GB) stars and investigate their kinematics and chemical composition. We find isotropic motions with no differences among the distinct populations. The comparison between the observed colors of the M92 stars and the colors derived by synthetic spectra reveals that the helium abundances of 2GA and 2GB stars are higher than those of the 1G by ΔY ∼ 0.01 and 0.04, respectively. The m F090W versus m F090W − m F277W color–magnitude diagram shows that below the knee MS stars exhibit a wide color broadening due to multiple populations. We constrain the amount of oxygen variation needed to reproduce the observed MS width, which is consistent with results on red giant branch stars. We conclude that multiple populations with masses of ∼0.1–0.8 M ⊙ share similar chemical compositions.

The rotation period distribution in the young open cluster NGC 6709

Open clusters serve as a useful tool for calibrating models of the relationship between mass, rotation, and age for stars with an outer convection zone due to the homogeneity of the stars within the cluster. Cluster-to-cluster comparisons are essential to determine whether the universality of spin-down relations holds. NGC 6709 is chosen as a more distant representative Pleiades-aged cluster for which no rotation periods of members have previously been obtained. This cluster is at a distance of over 1 kpc and has two red giant members. Isochrones place the age of the cluster at around 150 Myr, or approximately the same age as the Pleiades. Photometry is obtained over a multi-month observing season at the robotic observatory for STELLar Activity (STELLA). After basic processing, point-spread-function (PSF) photometry was derived using DAOPHOT II, and a suite of related software allowed us to create time series of relative magnitude changes for each star. Four time series analysis methods were then applied to these light curves to obtain rotation periods for members stars. We obtain, for the first time, rotation periods for 45 FGK cluster members of NGC 6709. We plot the Gaia DR3 colors of the member stars against their rotation periods and find a slow-rotating sequence with increasing rotation periods toward redder stars and a smaller clump of rapid rotators that have not yet joined this sequence. NGC 6709 has rotation periods very similar to those of another Pleiades-aged open cluster, NGC 2516.

Star chromatic number of some graph products

A star coloring of a graph [Formula: see text] is a proper vertex coloring in which every path on four vertices uses at least three distinct colors. Equivalently, in a star coloring, the induced subgraphs formed by the vertices of any two colors have connected components that are star graphs. A graph [Formula: see text] is [Formula: see text]- star-colorable if there exists a star coloring of [Formula: see text] from a set of [Formula: see text] colors. The minimum positive integer [Formula: see text] for which [Formula: see text] is [Formula: see text]-star-colorable is the star chromatic number of [Formula: see text] and is denoted by [Formula: see text]. In this paper, upper and lower bounds are presented for the star chromatic number of the rooted product, hierarchical product, and lexicographic product.

The shifting hues of Betelgeuse

Abstract How can you track changes in the colours of stars across hundreds of years, through differences in technologies, concepts and even languages? Dagmar L Neuhäuser and Ralph Neuhäuser show how ancient observations can still inform astrophysics today.

Galaxy Interactions in Filaments and Sheets: Effects of the Large-scale Structures Versus the Local Density

Major interactions are known to trigger star formation in galaxies and alter their color. We study the major interactions in filaments and sheets using SDSS data to understand the influence of large-scale environments on galaxy interactions. We identify the galaxies in filaments and sheets using the local dimension and also find the major pairs residing in these environments. The star formation rate (SFR) and color of the interacting galaxies as a function of pair separation are separately analyzed in filaments and sheets. The analysis is repeated for three volume limited samples covering different magnitude ranges. The major pairs residing in the filaments show a significantly higher SFR and bluer color than those residing in the sheets up to the projected pair separation of ∼50 kpc. We observe a complete reversal of this behavior for both the SFR and color of the galaxy pairs having a projected separation larger than 50 kpc. Some earlier studies report that the galaxy pairs align with the filament axis. Such alignment inside filaments indicates anisotropic accretion that may cause these differences. We do not observe these trends in the brighter galaxy samples. The pairs in filaments and sheets from the brighter galaxy samples trace relatively denser regions in these environments. The absence of these trends in the brighter samples may be explained by the dominant effect of the local density over the effects of the large-scale environment.